I believe there's a metal contact from the circuit board of
the watch that needs to touch the back of the watch for the
beep to work. If that contact fell out when you replaced the
battery, or you bent it so it's no longer making solid
contact with the back of the watch, the beeping won't work.

(Added misc.consumers back to the Newsgroups line, since I
read misc.consumers but not sci.electronics.repair.)