Hi,
ouch... indeed it's never a good idea to mess up Unity versions especially downgrading.
To prevent further similar situation, I strongly recommand using github or bitbucket ( Free for private use), and host your project with it, ALL my projects are on github and bitbucket, I never lost anything because it's all remote, so if indeed I mess up Unity version and corrupt my local copy, not a problem, I simply get the version back from the remote copy hosting on github.
Bye,
Jean