iTunes files appear corrupted?

Hi, I have all my iTunes files on an external HDD and for some reason today when I opened iTunes a lot of the folders and files appear corrupted.

First of all I get the exclamation mark saying iTunes cannot locate this file. When I navigate to the folder the music was in the folder names are now just a series of symbols and when I try to open them I get the error message 'the filename, directory name or volume label syntax is incorrect'. It seems to have picked random folders to do this to as 75% of my music is still there and plays fine.

I've Googled all night but had no joy solving this problem. Have I lost these files for good or can I repair them or get them back somehow please? As a side note, Windows won't even let me delete these rogue folders so I'm completely baffled.

Thanks, Lee.