What does this mean? "This computer is already associated with an Apple ID."

I can't play probably 90% of my iTunes music library. When I click any given song, a message pops up saying "This song could not be used because the original file could not be found. Would you like to locate it?" And then an option to "locate" is there.

Sometimes the message "You can download past purchases on this computer with just one Apple ID every 90 days. You cannot associate this computer with a different Apple ID for 42 days." pops up after the previous message pops up.

I am so frustrated!