I have an iPhone 3G which had a cracked screen. I replaced the digitizer last November and this morning the left third of it stopped responding to touch. What can I do to fix it?
This seems like a definite hardware problem.
You can take it to a retailer, send/take it to Apple, or do it yourself.
Here are DIY instructions and parts ($99)
The digitizer seemed to have been broken beyond repair so I did three things:
I made a dial pad app that moved the buttons out of the damaged area. (Note that I am a registered developer at the time of this writing and was able to install this on my device sans jailbreak.)
Went to the Apple store. They said that it would be $199 to repair, but they don't do repairs anymore. The only option was to replace the phone. They refused that too because I had put in a non-Apple digitizer.
I got an upgrade to the iPhone 4.
Aside from a hard reset or restore coming into the picture, reseating the display cabling (if you are still able to get inside) would be a good call...though if it was DIY and you weren't grounded while handling the components, then it's likely ESD damage catching up to you.
An unauthorized repair would put the unit out of warranty, regardless of its status, so an Apple Store is always available for service at a cost.