Did the button fall off, or did the mirror just pop off of the button? If the button fell off of the windshield, just get a glue kit from the local autozone or wal*mark or whatever.
I've only *pulled* mine off (to install an autodimmer), so I didn't have to stick the old one back on; I believe the procedure was something like sticking it on at an angle and then twisting it back into position -- someone else who's actually done it before can probably tell you.
best, whit