I have Multibit 0.5.18 installed on a Mac that I use occasionally (not my primary PC).
Yesterday I open Multibit for moving some BTC and I notice the blockchain is missing and I have no wallet on it. I leave the mac on for all the night, downloading the blockchain and today in the morning I come back to check the wallet. No wallet inside my Multibit. Seams like it was deleted the wallet and other stuff file form Multibit path. All BTC was gone. WTF is happened?!
Are you using Multibit Classic ? If yes then why you are downloading the Blockchain , As far as I know Multibit is a Lightweight wallet that dosen't need downloading the blockchain however for what comes to your problem , I had simillar issue once and wallets has been deleted . (But didn't have any money on them)
So my question to you is ... You used CCleaner or TuneUp utilities or any kind of those programs that clean your PC ? that could be the reason . you could try some recovery programs though and you should have your files back .