Found my multibit.key file, what now?
MultiBit developer here.

You'll need to use MultiBit Classic version 0.5.19 available as a download from the site https://multibit.org. This will allow you to open the .key file and synchronise with the block chain to recover all the funds associated with the private keys held in the file.

Once you have imported the keys, we strongly recommend that you upgrade to a HD wallet. The instructions to do so for MultiBit HD are here: https://multibit.org/en/help/hd0.2/how-to-upgrade-from-classic.html.

Essentially, you download MultiBit HD (the two can sit side by side without interfering with each other) and then spend from Classic into HD. 

Always keep hold of your private key files in case someone spends to an old address.