Exclamation marks next to songs in Itunes!

I can't play my songs/music files anymore in Itunes, they show an exclamation mark next to. I have my iTunes folders and files in an external hdd since i have no space in my internal hdd. I tried to follow an article where it says "restoring my music or download files, but it won't help since it also requires you to move back the files onto your mac and i don't have any space available.

Any help!