I suspect that someone, might have modified content type for the list and propagate the changes. This push-down modification can override any local changes.
To check this situation,
1. Open the list settings, and verify if you have single or many content type.
2. In list settings page, make sure that your column is related with the content type.
Best regards,
Riwut Libinuko
SharePoint Architect, Singapore
Microsoft MVP | SharePoint Server | Singapore | cakriwut@gmail.com
**Please let me know if you need more information**
Blog : http://blog.libinuko.com