It could be as a result of a recent update.
I have just come across this on another forum:-
Press Ctrl+Shift+Esc on the keyboard to bring up Task Manager.
Click File -> New Task (Run…)
Type explorer.exe and then press Enter.
If this does not work try the same again but just type in explorer.
If this does get you onto your desktop then you could try to repair the registry to repair the explorer.exe
Start - type in run
then type sfc /scannow
with a space after the c and before the /