Instructions
Hold the power button on the right of the PSP in until the unit turns off.
Turn the PSP back on and it should boot back into its operating system (OS). This is the first measure you should take. This overrides the PSP's power. If that does not work, move on.
Turn the PSP over onto its LCD screen and press the "Battery Eject" button. This pops the battery cover off.
Pull the battery out of the PSP. This kills the power and resets it. Place the battery back in and make sure the part of the battery that breaks the rectangular shape is up and to the right when inserted.
Turn the PSP back on and the PSP will boot into its OS and will not be frozen again.