What happened, Why and How much to fix?

Finally been out on my second hand mtb- a 12 1/2" Bronx Equaliser , 24" wheels, 21 whole gears (I feel your envy!)
Just prior to my first ride on it, I'd messed up my front and back shifters trying to adjust them to the point I didn't know what I was meant to be doing any more. So I finally caved in and took it to my lbs to be adjusted properly. I thought it might need new cables too but apparently not so. Nice, cheap job. When testing it out before home, I noticed the rear gears were now back to front (ie 7 seems to be the lowest) but not a biggie. Also, that I seemed to have acquired sort of 'half gears' where the gears wouldn't engage fully so you have to shift up or down a couple of times to engage the chain-rings. Assuming the bike shop people know a lot more than I, I went home and I've been out on it a few times okay and coping with the quirks.
This was my fourth ride- the last one was 20 miles round our flat park loop which went well. I'd cleaned and lubed the chain last night, checked my gears etc etc. I had put a new chain on the bike before the shop trip.
This time I was heading out of town on a rural bike path which is much more up and down. Firstly, my bike was making that annoying ticking noise when I tried to ride in the largest chain ring - why?
Then I discovered (by falling off my bike and into the bushes twice trying to get up small hills) that the front gears are now also labelled back to front.
On my way back, the bike seemed more noisy. Then while trying to change up from the smallest sprocket the rear derailleur just wouldn't move. Stopped and inspected. Nothing obviously blocking movement so decided to carry on- could still pedal okay. After 5 mins it magically started working again...More noise on the way home until I was turning into a street with a steep incline and shifting down (or trying) when my chain basically jumped off the biggest chain-ring outwards, ripping the screws out of half my plastic bash ring and folding it in half where it now rests gloating at me next to my tool kit. [blub]
Why did this happen? How much is it going to cost to fix it? How often do your derailleurs need re-aligned? 3 short rides seems awfully soon. I'm trying to get the bike through to next spring before I buy a new one but while I'm enjoy the rides, I feel like a mortal enemy of this ****ing bike [add the profanity of your choice].
Just prior to my first ride on it, I'd messed up my front and back shifters trying to adjust them to the point I didn't know what I was meant to be doing any more. So I finally caved in and took it to my lbs to be adjusted properly. I thought it might need new cables too but apparently not so. Nice, cheap job. When testing it out before home, I noticed the rear gears were now back to front (ie 7 seems to be the lowest) but not a biggie. Also, that I seemed to have acquired sort of 'half gears' where the gears wouldn't engage fully so you have to shift up or down a couple of times to engage the chain-rings. Assuming the bike shop people know a lot more than I, I went home and I've been out on it a few times okay and coping with the quirks.
This was my fourth ride- the last one was 20 miles round our flat park loop which went well. I'd cleaned and lubed the chain last night, checked my gears etc etc. I had put a new chain on the bike before the shop trip.
This time I was heading out of town on a rural bike path which is much more up and down. Firstly, my bike was making that annoying ticking noise when I tried to ride in the largest chain ring - why?
Then I discovered (by falling off my bike and into the bushes twice trying to get up small hills) that the front gears are now also labelled back to front.
On my way back, the bike seemed more noisy. Then while trying to change up from the smallest sprocket the rear derailleur just wouldn't move. Stopped and inspected. Nothing obviously blocking movement so decided to carry on- could still pedal okay. After 5 mins it magically started working again...More noise on the way home until I was turning into a street with a steep incline and shifting down (or trying) when my chain basically jumped off the biggest chain-ring outwards, ripping the screws out of half my plastic bash ring and folding it in half where it now rests gloating at me next to my tool kit. [blub]
Why did this happen? How much is it going to cost to fix it? How often do your derailleurs need re-aligned? 3 short rides seems awfully soon. I'm trying to get the bike through to next spring before I buy a new one but while I'm enjoy the rides, I feel like a mortal enemy of this ****ing bike [add the profanity of your choice].
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Parktools :?:SheldonBrown
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I'm a complete noob armed only with my bike maintenance book and this forum so I kind of have to trust the bike shop knows best.
Okay, so new plastic chain guard thingy and new gear cables- how much do you think it will cost?
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@The Beginner- I concede from what you say that the gears must have been back to front already. I have a terrible memory. I haven't ridden much and the last 2 rides I'd discovered a nice middle gear that worked well on gentle hills and the flat so I just suck with it until today when I needed more 'oomph'. Definitely gained extra shifts in the front chainrings between each 'bite'.
Should a bike click or have a loud rhythm coming from the drive chain? I remember my old low tech bikes as a kid did. I honestly don't know if others do these days.
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