I was having some problems with my cpu usage so a friend told me to empty out my c: drive onto an external hdd. I did and it worked. But an hour later it shut down and when I tried to start it up it said no OS found. Is there any way to get it working again? I don't have a windows 7 CD to re-install it. Everything is on my hard drive now. What do I do? :(

Time to reinstall your OS on C: drive.
What make/model is it as it most likely has a hidden restore partition.
This is normally activated during boot.

While it can be a good idea to have data and applications on a separate drive/partition, you can't simply move them and expect it to work correctly afterwards.

Hopefully you're learning that..