The ideal part would be if you'd have an older good version of the wallet or at least its private key. But I guess that's wishful thinking.

I would start with:
* check if you have latest version of MultiBit classic
* start without that wallet and do tools->reset blockchain and transactions. Stop Multibit, try to put that wallet again..

Let me try something else: did you rename some files by chance? or do you have by chance a folder there like 003-data or 003.wallet-data?

Edit: some ideas from here may help too: https://bitcointalk.org/index.php?topic=813278.0