Are all your tables in one file or are you linking to tables from other files?
Thanks for answering.
All the tables are in one file, which is on a FileMaker Server.
Used FileMaker 11 for years on Windows 7 and never had this happen.
I suggest running a recover on your file to check it for possible damage.
It's on a server which is looked after by someone else.
To run a recover, is this done from the file menu in this case, or is there a different process?
If it's hosted from FileMaker Server, download a back up copy of the file to your local machine and recover the copy.