Programs Disappeared

I installed many of my programs on my memory card and they worked just fine, but yesterday they disappeared and when I attempt to launch them, I get the error message, "The xxx file cannot be opened. Either it is not signed with a trusted certificate, or one of its components cannot be found. You might need to reinstall or restore this file."

Is there anyway to get them back without reinstalling?
Any idea how this happened? Can it be prevented?

Thanks for any advice!!!