You sound like there aren't many options here...But you might be able to press Ctrl-Alt-Delete and bring up the Task Manager, then click on the "Shutdown" menu in the upper left, then choose "Restart"... Upon restart, Windows tends to close down slowly and "might" save some of those files which were already copied over..

Still, the problem you are seeing is the reason that I tend to copy smaller chunks of files, a little bit at a time, instead of moving them all at once.

If all else fails, holding the "off" button in for 20 seconds should shut down the computer. You should be able to try again after restarting the computer.

Hope this helps.