Yikes, not good. I know when this happens on console, sometimes a hard-reboot (killing power) resets the connection to the download. Or, a complete re-download of the game...which ultimately sucks but if you're not moving at all at least you'll be making some progression.
Just done mine, at 21% its in an applying stage ie its applying the data you have already downloaded. However you should beable to see the actual data size falling and if it isnt then there is a problem.
I tried to repair but that stuck on 36% so in then end I re downloaded the entire game (thank goodness for superfast broadband - although it don't help with the ping or lag)
I have had this same issue. Sat all day yesterday at 20%. All morning so far. Have already tried other troubleshooting steps. Looks like it will be a full reinstall for me also.