When I try to play Skyrim, I get an error from Steam saying that the game is not available at the moment, and I should try again later. What's going on, and how can I solve this?
Simple solutions first: try restarting steam, or running the game in offline mode. If that doesn't work, try these other steps from Steam's support page for this problem.
Steam is a DRM System. You should read the linked article to understand what Steam actually is.
Your actual problem will probably solve by itself, since this is a temporary error.
However, if you want to speed up the waiting you could verify your game cache and/or switch content servers via the settings.