If you have two monitors:
Edit / Preferences, Network, Clear Cache
close SL & reopen
drag the open screen over to the *other* monitor
connect
Client / Character / Rebake Textures
If you don't have two monitors the most common case seems to be switching back and forth between SL and another proggie:
Client / Character / Rebake Textures first
Edit / Preferences, Network, Clear Cache next if you're still black
relog
Client / Character / Rebake Textures again (going into appearance can help nudge)
If it's still no good, you can try the workaround for a 'broken' skin:
If the skin is copy (doesn't say 'no copy' behind it in inventory):
* left click it Ctrl-C to copy, make a new folder, Ctrl-V to paste (this is so you can tell the copy from the original)
* change into a newbie skin (if you don't have them Library / Clothing, one of the outfits folders)
* Clear Cache, relog, rebake
* change into the *copy* of the skin
* Clear Cache, relog, rebake
If the skin isn't copy, but it is modify:
* Clear Cache, relog
* go into appearance, and change one of the sliders slightly
* go out of appearance, and rebake
If the skin isn't copy, isn't mod and if you see yourself fine with a different skin (the newbie one is a good test one), contact the skin creator and ask them for a fresh copy of the skin.
If none of the above works, I'm out of suggestions

.