using 3gs

Try the usual steps: restart, reset, restore.

http://support.apple.com/kb/HT1430

http://support.apple.com/kb/HT1414

If you try restoring from a backup and that doesn't fix the problem, try restoring to factory settings and, if the problem goes away, then synching your apps. You'll lose the app data and settings, but if the problem is due to a corrupt cache or settings file, that should cure it.

If none of that helps, then it sounds like you damaged or disconnected something while attempting to perform your own repairs. Apple will no longer provide support for an iPhone you repaired yourself, so you will need to either look for an independent repair shop to help you or try and figure out for yourself what went wrong and fix it. If that's not what you meant by "changed my back panel", then please post back and clarify.

Regards.