I was trying to export my design to a PDF (for print) when this happened (see first attachment). The elements in the two circles (far right) went missing (these circles should look like the circle on the far left - i used a red arrow to show what I'm talking about). And also, the (black) layers containing these missing elements were locked and cannot be opened/deleted/edited/moved.

After a few minutes, even the white circle disappeared (see second attachment).

Then the program finally crashed and gave me the option to reopen the recovered file but it was damaged (see third attachment).