Firstly, remove the game and inspect the disc for scratches, dirt etc underneath. Gently wipe disc clean, starting from the middle and wiping outwards to the edge of the disc. Avoid wiping in circles as this can damage the disc.

If this fails to work, the game's installed data may have become corrupt. Bring up the game menu on the XMB, and look for 'game data utility'. Select this and look inside for the data from the game that is giving you problems. Press triangle on the controller and you will have the option to delete this data (don't worry, it can be reinstalled once you get the game up and running again). Please note that this is installation data, not your saved game data, so your highscores etc. will not be affected once inside the game.