and the best way to do that is to reboot the machine.
Not sure why you think that all your work will be missing when the machine reboots though. Presumably you were saving each, and every, scan as you produced them and again after you rotated them.
However, if this is not the case, then yes, all your work will have been for naught.
The Finder, like Windows Explorer, is a very important part of the Operating System which will not work correctly if the Finder is not running.
Reboot is your only option
I'm logged in as a guest on a borrowed mac desktop, scanning in hundreds of photos on an Epson negative scanner, and using Photoshop to do very minor corrections such as moving the photo 90 or 180 degrees. I tried to move some of the scanned images onto a flashdrive, but then I started getting the swirling ball indicating a stall of some sort, so I force quit the application. Now when I try to open the finder, I get a "The application Finder can't be opened--10810." I then removed the flash drive--which I have discovered does not contain any files on it actually, just empty folders (I opened it on a separate laptop). But now I can't even open the Epson scanner program, or Photoshop, and there's nothing on the desktop of the original computer I was using. I was told that if I log out of the computer, I will lose everything, and I've spent more than 60 hours scanning, and so would greatly prefer not to do this by rebooting.
Does anyone have any suggestions on how to save all my scans and get back into these programs so that I can do more scans? I am assuming my scans are on the computer somewhere because at one stage where I was still able to use Photoshop, but the finder had disappeared, I could see that my image folders were still there when I opened a file.
I know very little about computers and macs, so any help is appreciated.