View App Store Download Progress (Downloading Maverick)
Try rebooting.
Quit App Store, open Launchpad, find the partial download of Mavericks installer and click it. This usually resumes a download that was interrupted/paused.
Reset App Store by clicking Reset Application from the debug menu, then reboot and try again.

You can enable the debug menu by running the following command:

defaults write com.apple.appstore ShowDebugMenu -bool true