First I would check all the flex cables are plugged in good, but before you could check and see if the flex connector cables are still in good condition. Sometimes when you aren't careful you can damage either the connector on the part or you could damage the connector on the logic board, if you damaged the connector on the logic board then you have 2 options, you can send the device away for board level repair or you can get a new logic board for your phone. But if that looks good then you can do some software troubleshooting by doing a hard reset of the phone by holding sleep wake and home button until you see apple logo then let off both buttons, if that doesn't work then try backing up your data up to iTunes and doing a restore of the phone. Lastly you could try doing a firmware restore of the phone using the following steps:
1.Hold the Power Button (3 secs)
2.Continue holding the power button and also hold the home button (15 secs)
3.Release the power button while continuing to hold the home button (10 secs)
4.Connect device to iTunes
5.It will say iPhone in recovery mode click restore
Hopefully one of these things will help, Good Luck.