If a physical drive in your PC (a hard disk, DVD drive,
etc) has gone missing from Windows Explorer and a reboot doesn't
bring it back, take a quick trip into the BIOS to see if it needs
to Auto-Detect your drive again.
The BIOS is the underlying system that works out what's
connected to your motherboard and if that doesn't realise that a
drive exists, then Windows will probably not notice it either.
To get
into the BIOS, reboot your system and, when the power comes back
on, press F2 or Delete repeatedly (once every
second) until the blue BIOS screen appears. As soon as most systems
start up, they will say "Press F2 to enter system setup" or "Press
Delete to enter the BIOS" or something similar to let you know
which key to use.
Use your cursor keys to select Auto-Detect Hard Disks
(or a similar name) and press Enter.
When that
finishes (after a few seconds), go to Save & Exit
(or similar name) and boot up normally into Windows.
(Note that, like the Windows registry, incorrect use
of the BIOS may have adverse effects on your system).