Partial Disk Encryption- wont resume

I was encrypting an external disk (large one, taking a long time) and (probably because my computer did an update during the process) I now get an "Encryption paused" message when I check the desktop. If I hit the "resume" button I get a box which says " Encryption Desktop is currently processing another disk. Please wait before starting another encryption process.". I am NOT running another encryption. How do I get out of this loop?