Recently when I try to open certain sketch up files I get a prompt that says “file is locked. open in read only”. When I open the file and pull dow the unlock menu the two option are greyed and hence unavailable to me. How did I lock it in the first place and how do I unlock it?
That message generally has to do with SketchUp detecting that the file is already open on the computer. You might try doing a cold reboot on the computer and then open the SketchUp model file again.
You could also go ahead and open the file, then use Save as and give it a new name.
The Lock/Unlock thing in the Context menu has nothing to do with this problem. Lock/Unlock is for locking objects within the model, not the model itself.
What version of SketchUp are you using? What operating system? Please complete your profile.
In addition to what Dave said, a common case where you see the locked file message is when it is already open at the time. Try double-clicking a file to open it, then go back to File Explorer and right-click Open the same file. The second instance of SketchUp will report that the file is currently locked by another user. That other user is you. If you see the locked message, point at the SketchUp icon in your taskbar, and see whether the document is already open.
Windows has some glitches around this issue. You might also get this message if you have the Preview pane displayed when viewing your folders and files in Windows.