@gerryo - this is a four-year-old thread and we got no feedback from the original poster (OP). We don't know what actually happened with the OP's problem. All we can tell you is that it is one of a few possible things.
1. Some type of corruption. To fix this, start by making a backup copy. THIS STEP IS CRUCIAL!!!! Do NOT proceed with anything else until you have a good backup copy. You don't want to lose anything if the fixes go wrong. After that backup has been made (simple copy/paste to a convenient other folder is all you needed), try these:
1.a. Simple corruption: Perform a Compact & Repair. Since you have that backup, if the C&R fails, you can get back to where you are. If the C&R works, problem solved.
1.b. Another type of corruption of compiled code... see the DECOMPILE option in the link below. It is about the 4th section title in the linked page.
Decompile and Compact Your Microsoft Access Database to Improve Performance and Fix Corruption
www.fmsinc.com
Don't forget that you need to manually compile after you do a decompile. The FMS, Inc. article explains how to do that.
1.c. The form is REALLY bollixed up. From the External Data segment of the ribbon. try to export the form to a text file, which is permitted in Access. Then delete the form, perform a C&R, and reload that form from the text file you just created.
1.d. Since you still have a backup, delete the form from the current copy and use the External Data section of the ribbon to try to import the form from the backup. It MUST be done as an IMPORT from the backup since trying to do an EXPORT from the backup might exacerbate the corruption. Direction of data flow IS important in this case. NEVER direct any actions INTO your backup, since it is your lifeline.
1.e. Again, since you have a backup, delete the form and attempt to manually recreate it.
1.f. If this ALSO fails, immediately create a blank database and use the backup copy as a source target for a complete import of everything (in the backup file) to the new, currently empty file. Things that do not successfully import are indicators of severe types of corruption.
2. You got caught in patch Hell. Use the Start >> Settings >> Update >> Update History path to see if you had a Windows Update just before your problem started. I'm actually including this only for completeness because from your description, it is too selective to be a general patch disorder. You said only one form was affected. But if nothing else works, you could try to roll back the patch to see if that helps.