I doubt it has anything to do with the 2.3 upgrade or the font hack. Something else is going on.
From the
Kindle Troubleshooting Guide
Quote:
Kindle screen frozen or Kindle unresponsive.
Possible cause:
* Low battery charge.
1. Plug the Kindle into a wall outlet.
2. Ensure the Kindle is charging (the indicator light should be on).
3. Wait 2 minutes.
4. If necessary unplug the Kindle and reset by moving and holding the power switch for 15 seconds before releasing it.
Kindle showing lines on screen.
Possible cause:
* The Kindle may be affected by electromagnetic interference or screen may be defective.
Move and hold the power switch for 15 seconds before releasing it.
|
If neither of those work, you're going to have give Amazon a call on this one.

They're really good with working through issues like this. They'll probably send you a replacement Kindle if they can't figure it out.
Good luck!