You should first:
a) do NOT use the drive any more, then
b) SCAN the drive with a program like EASEUS Data Recovery Free:
http://www.easeus.com/datarecoverywizard/free-data-recovery-software.htm
You probably need a PAID version to recover the files but this should SCAN to look for them. When you RECOVER the files you need another drive with enough capacity to COPY the files over.
(no need to buy the new drive first. SCAN first. as long as you aren't WRITING you won't overwrite any file data. Also, once you scan it should tell you how much data it found)
If it can find them (which it probably can unless the drive is physically malfunctioning) they won't likely be in the proper folders so you may need to reorganize everything.
If you recover them successfully you should then:
a) do a LOW LEVEL FORMAT on the problematic drive, then
b) do a FULL FORMAT (NTFS), then
c) run a low level diagnostic on that drive
If the last part is confusing, just wait and ask later. (Basic NTFS format doesn't wipe everything so there can be PROBLEMS that remain. A low level format wipes everything off the drive so you end up REBUILDING the file table etc.)