I have a sheet that I have used for years. Today, I can't add new data. I tested a new document and it seems to be working. What could be wrong?
Do you get any specific error? Try this, copy the sheet with a different name and see if you can add more data in the new one.
Can you show us a screenshot of what you see when you open the sheet? (Make sure to blot out any sensitive data there might be on the screen before posting here.)
I think you have lost the file history, I hope there is nothing important saved there, so try reinstalling the program