ITunes/iPod library problem... i imported the wrong library!?

okay, so once in a while, my itunes says that i need to pick a library, dont ask me why, but i had like 357 songs on it and now i picked the wrong library and i dont know how to get my old one back. now i only have 263 songs and i wnat my other library. how do i get it??? i cant really find it on my search thing but i just want to know how to switch libraries.