iTunes help

Today while i was on the computer, listening to a song on my itunes, i got a pop up saying i had too much music for itunes' memory and it could not save my library. i clicked "okay". when the song i was listening to finished playing a new song didn't start up after it, so i clicked on the itunes icon on my dock to see what was wrong. but when i clicked on the icon my itunes wouldn't open. it says that my itunes is in use, but the window with all my music just won't show! and i can't quit out of itunes because my computer doesn't even recognize that i've clicked on it. i'm thinking this has something to do with the little notification i received but i have no idea why or what to do. ???