There's a thread about the same issue posted several months ago. You can have a look at it here. Basically, the thread says that it could be a problem on Microsoft's side (especially if is happening to more than one app). A proposed solution (which other users have found to work) was to go to windowsphone.com using a PC web browser and install the app from the app page (click on "Install" or "Reinstall"). The app should then start downloading in your phone.