Is your Fat/Slim PSP Bricked or you have deleted the Flash0 Folder by mistake don't worry follow this Guide to get your Flash0 or an Bricked PSP Back!
Do you have a PSP Slim? If so the only solution you have is using Dark_Alexs Universal Unbricker which you can get HERE. You will need to make a Pandora Battery first then modify the magic memory Stick with the files above.
If you have a FAT PSP then folow the guide below...
You will need to go HEREand download the recovery folder.
Go into Recovery Menu and toggle the USB so you can connect the PSP to PC via USB.
Now extract the recovery file you downloaded and drag the RECOVERY folder it extracted to the /PSP/GAME folder.
Now disable the USB toggle and then go to RUN PROGRAM AT/PSP/GAME/RECOVERY/EBOOT.PBP
This will recover you back to 1.50 firmware. You will now need to upgrade to 3.52 M33-4 then 3.71 M33-2 custom firmware (If you happen to have a Pandora battery it would be quicker using the above universal unbricker to get back to 3.71 M33-2)
After you get updated back to 3.71 M33-2 make sure you back up your flash0 properly. Below is a guide for doing this...
The correct way to backup 3.71 M33/M33-2 files is by enabling flash0 via Recovery Menu (Don't flash your XMB via VSH menu or you will semi-brick).
When the flash0 window comes up on the PC go to Tools> Folder Options> View Tab>...
Tick the radial that says (show hidden files and folders)
Untick the box that says (Hide Protected Operating System files).
Click on Apply then OK.
Now drag the flash0 files over to PC to back them up (Make sure the steps above are also done when recovering your flash0 files).
make sure your using a custom XMB that is comaptible with 3.71 M33-2 custom firmware.