Out of memory errors are pretty serious and can cause you to lose the ability to save or edit the open file. Try exporting to IDML for safety.
If nothing works, the best chance you have of recovering the work is to force quit InDesign, then reboot the computer and launch InDesign again. If the recovery data is not corrupt the file should reopen automatically.
As for a cause, it's very hard to say, but it could be a corrupt image or link, lots of pasted graphics, or something else entirely.
Yes I have cut and pasted a lot from another Indesign file.
Is there another way I can place one InDesign file into another?
Copy/paste from one ID doc to another should not be a problem (but you can use File > Place... to place an entire page, or pages). The pasted graphics was more about pasting from Illustrator or Photoshop.
Wow. I did not even think of that :-) Thank You :-)