Well my only guess is either its corruption or something has changed (you just may not know it) based on some kind of structural/calculation issue.
Can you go thru back up copies and find one that works?
"starting a new survey" would require a script to generate response records in order for the respondent to enter their responses? (There is more than one way to set up a survey for new responses, but this is the most straight forward option.)
both a problem in the script or in the data used to match question records to particular survey could cause such to happen.
A damaged index could also be at fault--advanced recover options can repair all indexes in a file.
Turns out some of the questions were missing from Questions. Not sure how that happened.