Try pulling out the battery and placing it back in, then start the system up. It should bring you back to the "Sony Computer Entertainment" logo.
power the psp off but instead of just hitting the switch for a second keep it all the way up for abot 10 seconds machine will shut down and you can just power it on as normal the next time round
hold the power until it reboots. I think its like 10 secs??
There is the standard ATX "hold the power button for 5 seconds" until it turns off and then restart it or you could just yank out the battery... your call.
have you tried removing the batter for a minute, then putting it back it, sort of like a hard reboot?
maybe you can just pull out the battery from back ?
I've never experienced this problem myself, but I would try the following steps.
1.)turn off the PSP
2.)remove the battery
3.)remove the memory stick
4.)replace the battery
5.)turn on PSP
I would guess that it should then boot up to the menu screen. After It gets to the menu screen you should be able to put the memory stick back in.
Hope this helps.
Easiest way to get it free is to simply remove the battery from the PSP for a couple mins then put back in. Should be free at that point. I haver had to do this several times while experimenting weith the different patches, etc..