What phone is this - guessing a 3g or 3gs? I second the previous poster's suggestion. Open it back up, remove every thing and carefully place it back making sure everything is firmly connected. Pay close attention to the battery and the 3 connections from the glass/ display to the logic board.
If you're questioning if you got #3 in far enough, I like to use a piece of tape (I use kapton but scotch tape will work also) to firmly secure those pesky zif connectors. With the locking bar up and unlocked, seat the cable, then place a piece of tape on the ribbon cable to get a good grip and pull the connector all the way in.