We accidently erased a group on files

If you erased the files in the project media, they are still on your harddrive unless you choose "remove from project and delete files" .. Then it's a question of retrieving the files on your harddrive. This is possible if you have not written too much to that drive since the deletions. There is "undelete" software that could do this for you. I don't use any -- but I'm sure others here do. Here's a link about Undelete