..Black screen but full function..
I don't have a 3088, but here are some thoughts..
1. Yes, maybe the update failed. However, I would expect the device to be smart enough to recover from a partial update. Standalone devices normally use checksums and memory switching to verify updates and avoid failures.
2. Maybe the update has set the screen brightness/contrast to a bad value, hence the black screen. Is there a menu option to change it? Obviously you'll have to guess the buttons if you can't see the screen.
3. Hardware failure. Has the screen or the LCD connector simply failed?
Also, try removing the batteries from the handset overnight. This will allow the circuits in the handset to discharge. This can sometimes cure "memory corruption" problems.