This is happening alot with Multibit HD. I tried uninstalling the wallet then reinstalling but that didn't work. Since the wallet didn't ask me for a new password I just assumed the password was stored on the system.

I am running Win 10 so I ended up navigating to C:\Users\[your username goes here]\AppData\Roaming and deleting the Multibit HD folder. When I restarted Multibit HD it prompted me for a new password. I was back in after that. This has happened to me 3 times so far. Best use another wallet.