File as read only (?)

When I need to open a drawing, I get the attached warning. It is in personal computer and I am the only one using SketchUp and the file is not open.
Any idea why this is happening?
Thank you in advance.

Have a look here.

Did close and reopen SU? Ive noticed a bug where SU will ‘close’ but still have the process running. Check Task Manager to see if this is the case.