I have a removable hard disk (100GB) that I have been using for about a year now. It was 96% full last night but today I opened it and it wouldn't open. I have been sitting here for like an hour and it won't start, though my PC recognises it. Through some investigating I found that my disk is now 100% full! How could that happen??
A friend told me that perhaps this happened because of the thumbnails of various pics I was going over last night, but it is essential for me to open it since my work of the last three months is there and I need it desperately.

Please please can anyone help me? It is essential to open that disk!

I opened it and it wouldn't open. I have been sitting here for like an hour and it won't start, though my PC recognises it.

Could you be a bit more specific? What does "it wouldn't open" mean?

If your PC recognises the hard drive,why not delete a few files,and programs that you don't really use to get some space back,then defragment the drive,and reboot your PC.Dont just delete them from whatever your drive folder is called though.Use start,control panel,then Add and Remove programs from Windows.
Don't forget to regularly check your hard drive for viruses,spyware,malware,and the usual Adware nonsense.Another good program for removing junk is Crap Cleaner.Its freeware,and can be obtained from this link http://www.majorgeeks.com/download.php?det=4191
You will be amazed how much junk your PC stores even when you've simply been browsing the web.Also use that at the end of the day.

Just a footnote,when you say you opened it,but it would'nt open,do you mean you can't access any files,or will no programs work?
Just see if you can get any of the advice I've given you above to work.If not,post back and let the forum know how your getting on.

All the best,
Nez.