[DJ Johnny Rash]

First off, what you SHOULD have done was back up your Flash0.
Looks like you're pretty screwed.

But there are some things you CAN do. (Good thing you have your handy recovery menu)
Unfortunately, I was not able to find the recovery files for 3.90.
Google did not smile upon me but maybe it will smile upon you.

But from my knowledge...you have 2 solutions.

Option 1:
Locating or requesting a clean dump of the flash0 of a 3.90 PSP.
However, there is a possibility this may not work.
Especially if you have a Slim.

Option 2:
Pandora's battery.
If you can get a hold of one...good for you.
You just saved your ass.

You can learn more via Google.