If you've had the wheel out of your bike doing some work....

t4tomo

Always remember to reconnect the brake caliper.

I fitted a new chain a week or two back on my little used "station" bike. Didn't want the Brompton with me today, so took that instead and wondered why the back brake wasn't working! Luckily I hadn't been doing any work that needed the front wheel out too so no harm done.