Cant Open A .wallet File - HELP!

[Fishbones78]

Hi
I've got myself a multibit.wallet file for MultiBit Classic that just won't open, with the reason 'null'. I literally had it open 5 minutes ago before MultiBit crashed when importing more old private keys.

Anyone know what format the file is in, and I would assume the private keys are in this file as I don't have a plain text copy?

The address only has $10.00 so no biggy, but they will be lost for eternity otherwise (poor Bitcoins!)

Thanks