I'm working on RStudio for more than a year and it's the first time I have this issue. Since I updated RStudio to the latest version, I cannot acces my r projects made before the update. When I try to open them, I only get a grey screen, as if RStudio couldn't load the project.
Try resetting the project-specific state - you can do this by navigating to the Project's folder in your file browser, and deleting the .Rproj.user directory there. Have in mind that it is a hidden folder.