I recommend this step, also… but first, make a backup of and/or move your “Unreal Projects” folder. Then choose “Remove” from the dropdown menu for UE4.11 in the Library tab of the Launcher. After removed, close Launcher. Delete the UE4.11 directory and make sure you didn’t install it on another drive as well. Then, in the Launcher, choose “+Add Versions.” After reinstalling UE4.11 and verifying, open a blank blueprint project with starter content and see if you can access these assets. Then move your projects back, one at a time, to the newly created Unreal Projects folder, checking to see if you can open project and assets with each move (closing the Launcher and Editor between each move.)
If you can successfully restore two projects this way, then close the Editor and Launcher and move the rest back all at once. Try these steps and let us know if you were able to reinstall UE4.11 and restore your past projects.