Argh!

are you downloading more than 1 game?

Try restarting steam, or changing your download server.

You can try changing your server, but during the sales this is a regular byproduct of them getting hammered all the time.

Settings - Download Tab - Download region

Maybe your HDD is very busy, or low on space and it's constantly defraging? HDD could just be dying? Something to think about.

Are you playing another game while it's downloading? Steam's default setting is to pause downloads while playing a game. Everyone else has suggested the usual fixes.

Happened to me recently for another game too.

Its been happening to multiple people today. I assume its caused by the sales and all the people wanting their new games. Just need to be patient.

Too many people are downloading on steam. It happened to me, and I changed the Download Region (Steam > Settings > Downloads > Download Region). You'll have to try many of them.