Unity project weird behaviour - Scenes not opening / script links not working

I opened up a project I had not opened for a couple of weeks today and after hitting play I just got a black screen.

There were no errors in the console so I had a look at the project.

A quick look revealed that all the links in the project have been lost. Links to scripts, sounds textures etc.

In the inspector is says Missing(Mono Script) or Missing(Shader) etc. When I try to relink them it changes from "missing" to "None". Even though the assets exist in the project they can not be relinked.

The editable variables in the scripts are remain visible even though the link to the script seems to be broken. But any GUI textures are missing their links and they also change to "none" if I try and relink them.

If I delete one of the script components and reattach it, it seems to reestablish a connection with the script ok but then I still can not link GUI textures...

Another quirk with the project is that I can not open scenes by double clicking them in the project window... I have to go to file > open scene for it to work.

Ahh! I have no idea what is happening here. I have multiple old versions of the project which still work but I will loose a couple of days work to go back to the most recent working version...

This is a really strange problem. Has anyone come across anything like this? any tips, advice for getting it working?

Thanks