depends on the stem, it's often marked on it
i'd try taking the bars off, cleaning all the clamping surfaces, stem, bars, faceplate, lightly grease the threads of the bolts, reclamp
when clamping, do it a bit at a time and try to even the force over the faceplate
for instance, if it's got four bolts, viewed from front...
1 - 2
3 - 4
...do them up in this order 1 4 2 3, start finger tight, then give each a half turn until you reach enough force to clamp the bars, 5-6Nm is usually plenty
oh, and have a good look just to be sure there isn't a crack anywhere!