It's one of the following -
1. Either the .cso is corrupt or the rip by the uploader was bad and you need to download again.
2. Some .iso's don't run properly as compressed .cso's so it needs to be converted back to .iso through a program such as PSP Compressor.
3. Change the driver mode on you psp by hitting the select button and choose between m33, Sony NP9550, or OE isofs.
4. In a rare case, your firmware might've been corrupt while updating - if all of the above fail, try updating your firmware.