Unity does not recognise any assets.

Hi, I have had unity for a while now, and was in the middle of a project when unity updated. Now all the assets in that project are invisible for the editor, even though they are still present in the project's asset folder. All the standard assets are also invisible, so when I create a new project, there are no standard assets to import. They cannot be imported from the editor menu either. Please could someone explain what is going on?!