Since you can't use the task manager lets try the registry method to fix it.
Try these steps.
Press Windows+R ( to launch Run.exe)
In the Run program, type there regedit and hit enter
Then in the Registry Editor program. follow this folder path route
HKEY_LOCAL MACHINE / SOFTWARE / Microsoft / WINDOWS NT / CurrenVersion / Winlogon
Now click on Winlogon and after clicking then look right side pane.
Here you have to find the Item called "shell" and click on twice (Double click)
In the Value Data Edit box, Type Explorer.exe (The value od this key "Eplorer.exe) then click on OK
Now restart your computer.