Can't open the illustration. The file is locked or in use. ID:-54

Please help me with this error in illustrator.

Can't open the illustration. The file is locked or in use. ID:-54

All files on the desktop (except those that were edited two days ago) will not open due to the above error.
The file I edited a week ago won't open either. pdf and png file saved as the ai file was not opened too.
I don't use cloud storage and drop boxes, and they're all files on the desktop. I couldn't solve it even when I referred to other solution in the community. 
Please help me recover these files..