Okay, unfortunately that usually means LC can't recognize the file as a stack, which sometimes means the stack has been corrupted. If you have an earlier backup, see if that opens. If so, you'll need to add in the things you've done since that backup.
It's very rare that stacks get corrupted like this, so before you panic make sure the stack isn't opening invisibly. Does it show up in the project browser or the Windows menu? If not, then sometimes the LC team can correct the problem but you'd need an account that includes tech support for that.