sorry to hear that, just as the guy above said, try leaving it over night. if it cannot work, i am afraid that u have to buy a new memory stick for ur PSP. i had the same experience too, at last i had to got a new one from this site:
http://www.sourcingmap.com/advanced_search_result.php?searchbar=1&keywords=psp+memory+&searchType=all&x=0&y=0
it works fine till now! maybe u can have a try.
jennyyu, August 2007