How long have you had 3.90 m33 installed?
Have you put any ctf or flash themes on it?
I've had that problem happen to me when putting flash themes on the PSP and it didnt like it (
).
Are you able to get into the recovery menu?
If it is to do with a flash theme, replace the original files, if it isn't, you need to recover the PSP, I'm not too sure right now how to do that on a PSP with 3.90 M33 but I will find out............................................... ....i think though, that you would need to put the 3.90 m33 files onto the psp, but instead of naming the folder m33 or whatever folder (which goes in the GAME folder), put the required files into a folder called RECOVERY and then in recovery mode, tell the PSP to "Run program at /PSP/GAME/RECOVERY/EBOOT.PBP - which will reinstall 3.90 M33 for you.