What changed just before you saw this difference?
One possibility that suggests itself to me is that your field may now refer to a different field in the layout's underlying table than it did originally. If it now refers to a number field with a numeric values only validation field option, this would explain both the change in values and the appearance of this error message when you try to select a check box value.
If that's not the case, you may want to put a copy of this field on your layout temporarily formatted as an edit box so that you can see the actual data entered in this field. With two check box values, you may see up to two values separated by a return so that each value is listed on a different line. Checking this added copy of the field can confirm whether or not the data in the field was somehow changed.
And you can go to Manage | database | Fields, find this field and use the validation tab of field options to remove this validation option if it is not what you need/want for this field.
The only thing new is that I added a a new checkbox field on Wednesday. I have now deleted that addition. Is there a way to revert the check boxes that become unchecked?
if no, I have about 50 files since the incident occured. Is there a way to export the new files into a backup or do I have to re-add them to the backup of FM?
I think that you mean "tables" when you say "Files". They aren't the same thing. I've replied to this in your other thread.
But since we don't know exactly what happened here, it's hard to say what options other than restoring from a back up file might work. Even importing all data from your current file into a clone of the backup file may not fix this as the data may have been deleted or changed in your current copy and then importing this data into the clone is just a case of importing the data with this info missing into your clone and then you get the same results.
But to repeat from my last post, the change you describe a field that now appears blank and an error message insisting on numeric values suggests that you may have a layout field object that now refers to the wrong field.
On the other hand, if this was field originally formatted as a drop down list or popup menu and you are switching to a check box format with a different value list, it is possible that your original value list enters a number into a number field, but displays corresponding text associated with that number value.