I had This Problem 2 months ago ...and well ..It Was Pretty Dumb..All you have to do is ..Go to
1) On the Psp
2) Update Your PSP
If you Problem Still Presists..Then do this....
1) Reboot your psp (hold down the power switch for 10 second, then turn it on)
2) As it's booting up hold L + R to get into recovery mode
3) From there got to Advanced => Advanced Configuration and disable the "Execute BOOT.BIN in UMD/ISO" option. Exit the recovery console and boot up the game.
You should be good to go.
And Your Welcome..