When you say, "work wasn't saved", do you mean you never saved that project or that you worked for a while without saving, but the project had previously been saved?
That is to say, is there a chance that you have an autosave of your file?
Also, what, exact, version of AE are you using? What OS? What happened when you got the error message? What were you doing when it happened?
Also, check out this thread: unbalanced unlock ( 7 :: 40 )
Are you still facing this problem? If not, let us know how you solved it. If so, please let us know so we can assist you further.
This solution did not work for me. I'm still currently experiencing it, although I'm able to save after I hit OK. But if I so much as move the playhead on the timeline it pops up again. I'm running in a large sequence 7000x750, but just trying to place regular jpeg images in the frame. So far there are...8. Nothing majorly taxing. I've a robust 2013 Mac Pro, 32GB RAM, macOS High Sierra.
I do have some reszie expressions on two of the photos to conform their height to 750. It was making things act sluggishly, so I wonder if that has something to do with it.