explorer.exe is the Windows “Explorer”, a tool that makes it easy to find and manipulate all kinds of information about a system, including its registry and Windows API calls. During system shutdown, explorer.exe looks for and executes windows shutdown.exe. When shutdown completes, the shutdown.exe program executes exit.exe, removing most of the explorer.exe registry entries. Once shutdown completes, explorer.exe stops executing.
After a system shutdown, explorer.exe starts executing exit.exe, which automatically kills explorer.exe. This is the reason your computer will not boot up at all after a system shutdown – it’s still executing explorer.exe.