For all who can't launch DOTA right now, follow these steps to sort it out...
Start steam go to settings (downloads + cloud) untick "Enable steam cloud synchronization for games which support it" shut down steam and wait for the exit now to clickable and click it.
open steam
Go explorer and go to your steam folder
Rename the folder "appcache" to what ever you want
shut down steam
and restart steam
Wait for the pictures of your games load. Could take 1 or more mins.
AGAIN untick "Enable steam cloud synchronization for games which support it"
Then you can play games.
For those of you having troubles launching Dota2.
Found on reddit