I have no idea, could be any number of things.
Has anything been updated on the site recently.
The first thing to do is to start deactivating plugins to see if there is some type of conflict. After that it would be testing with a different theme.
You’re going to have to narrow down what might be causing the issue.
This is just a thought, but open the field editor for a field group that is having this problem and make sure the field keys have not been altered somehow. Click on Screen Options and the top of the editor and check Field Keys.
Field keys should be in the form of “field_XXXXXXXXXXX”
