Explorer.exe might not be running/is set not to run/is corrupted. If you can, do CTRL + ALT + DEL, select task manager, and start the process. If it doesn't work/if it doesn't work on reboot, then do the same thing, but go to File > New task, and type "CMD" and check the run as administrator box and hit ok.
After an Admin. command prompt opens, type:
and press enter. You can also use a system restore (if you can get explorer.exe to pop up by running it manually then holding SHIFT and clicking restart) or a refresh. (<A refresh deletes all programs but keeps your files)