It's almost impossible to overwind a clock. If you try, the key will usually break before the spring snaps.
If the springs are dirty, then winding it beyond a certain point will stick the spiral spring together and stop them working - a common cause of this is tobacco smoke.
If this is likely, squirting WD40 into the spring will probably help.
Releasing the spring requires a special tool, and to do it safely requires a certain amount of skill, so it might be best to take it to a professional repairer - you can find one here:
http://www.bhi.co.uk/