Press ctrl+alt+del and under Processes end explorer.exe. Then go to Applications, New Task.., type explorer.exe click OK and then your desktop should show up.
To backup a selected branch/key in the registry, do this:
Locate and then click the key that contains the value that you want to edit
On the File menu, click Export.
In the Save in box, select a location where you want to save the Registration Entries (.reg)
In the File name box, type a file name, and then click Save.
Click Start, Run and type regedt32 and press Enter.
Navigate to: HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\winlogon\
The Shell variable in the right hand pane should = Explorer.exe
Navigate to: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\Winlogon
In the right pane check the value of the Userinit key.
In the right-pane, the value of Userinit = C:\WINDOWS\system32\userinit.exe,
Type the above value exactly as given, including the comma. Also, change the path to userinit.exe appropriately if Windows is installed in a different drive.
Close Registry Editor and restart Windows.
If that doesn't solve the problem have a look here.
https://www.techrepublic.com.com/5208-6230-0.html?forumID=101&threadID=283728&messageID=2683528
Let us know if it is helpful.