- open Titanium Backup
- click Backup/Restore
- scroll down and select Wi-Fi Access Points
- click Restore next to the most recent backup
- reboot your phone
of course this will only work if you've already rooted your phone, installed Titanium Backup, and backed up your wifi access points. if you haven't done that, there are several apps in the google play market that claim to fix your wifi. if those don't work, you may need to a factory reset.
the moral of the story is: use Titanium Backup to back up your android device often!