You may want to give the disc a bit of a wipe- I've found the cover getting messed up with fingerprints will make the discs unreadable over time. Give the actual disc a quick wipe as well using a soft cloth going from the center out.
If it still doesn't work after that you may have to contact Sony regarding this. If your PSP is under a year old you're covered under warranty- you may need to say it was a gift if you don't have the receipt.