I have a USB hard drive that shows up as the F drive. But i accidentally disconnected it and now when I plugged it in again it turned to the G drive. Now the F drive has a CD drive icon and it's empty.

How do I turn the hard drive back to normal?

I've tried the System Restore but it said it couldn't change the F drive because it was diconnected or unplugged.

You can reassign drive letters by using Disk Management. Go to Start...Run...and type in "diskmgmt.msc" (without the quotes) and press enter.

Right clicking on each drive will let you change the letters to any letters that are free. So, for example, if you want to swap the F and G drives you'll have to:
1) Change the F drive to H (to free up the F drive letter)
2) Change the G drive to F
3) Then change the H drive to G

Also, please be aware that changing the drive letters may affect the ability to run the programs on the drive.

And finally, this doesn't address the issue of what this F drive is - why would your system just add an empty CD drive? Could this have been the result of a program that you installed recently, or could it be the result of a virus?