Since you mention a coffee cup, it appears you are hosting the file over your network. Hosting from FileMaker Pro or Filemaker server?
It's possible your file is damaged. Do you have back up copies?
As a test, you can try using Recover to recover the two files and then host those copies to see if you get the same issue. You may want to save a clone of an older back up copy of your file and import the data from your current copies into these older copies and then put them in use. This not only may eliminate any damage to your file, it will rebuild the indexes in the files and this index rebuild may solve your problem.
Thank you everyone! I was actually able to figure it out with my IT dept. The "defined relationships" field got messed up. I had to set it all back up again