Why is Unity deleting my prefabs!?

The other day, I had finished doing some work on game. Everything worked just fine. Then today, I got on and a lot of my prefabs were gone, my tags were deleted, and it switched my platform! I wouldn’t be so mad if this was the only time this has happened, but it’s happened multiple times. Also, some things don’t work sometimes… For example, I have a menu that pops up when you die. Just today it started spawning off center (but only sometimes!). All I did was open Unity and hit play. Everything works just fine on my iPod test version. I have no idea what I could do to fix this issue. I don’t feel safe continuing to work when everything I do could just disappear the next day. I have not changed my project location. I’m on Unity 5.0.1 and on a mac. Any help is appreciated, thanks.

I was a able to fix most of my issues from the solution that @bobin115 gave. By going to Assets - Reimport All I was able to get my prefabs back. Sadly my tags are still gone, but that is a much easier thing to fix:)