How old is the watch? The buttons could have a build up of dirt around the buttons and that will keep them from working! If thats the case you can take a pin or a very thin screwdriver and run it around the botton to unclog the build up...or you can also take it to a reputable watch repair shop and they can disassemble the watch and remove the buttons and clean it really well.

Its also possible that the watch wasnt assembled correctly when it was closed after the battery change as well. Some movements have a small metal flap that the buttons push against to work the functions. If the flaps get moved then the button might be pushed in and stuck since the flap doesnt push the button back out...OR...there is a tiny little c-clip that holds the button in place and if thats removed then the button might not push back out or could come out of the case all together!

So...check around the button for a build up of dirt first, try removing the dirt with a pin or small screwdriver. If the button comes out then put it in a plastic bag along with the watch so you dont get any moisture inside and take it to a reputable watch repair shop! :-)

Good luck!