I've just had this happen to a single QVW file, and in this case the file was corrupted. Not sure how it happened, but opening the file in a text editor the file appears to only contain blanks, where it should have binary data followed by an XML definition of the file.
It's a case of reverting to a backup, if you have one, or recreate from the project folder - again if you have one.
My best guess of what occurred is an interruption during saving.
It happened to us and created a case with Qliktech and there was no resolution but having a backup of your qvw, anyways there are some recomendations:
Unfortunately, I was unsuccessful in opening the QVW. The only solution is to use a backup of the QVW. I apologize but we don't have any methods to recover the QVW. Corruption can be caused by a lot of different issues. It can be caused by other software programs touching the file like antivirus. It can also be caused by network issues. It could be caused by a failed saved QVW.