Where did you download it from?
Did you try and download it again?
Did you try and download it from another source?
he downloads from Steam, i believe.
well, this thing happened to many people, havent happened with me though, i experienced stuck download, but it went on again after some time.
some possible fixes:
-change the download region
-verify integrity of game cache
-change your internet connection (not practical, but fixed some people's problem), or the very least use proxy/vpn/tunneling
-check your harddisk (using S.M.A.R.T test) and ram (too much overclocked, or need more voltage, or simply, failing
)
-scan for virus/malware.. possibly malware alters the files?
hope you can fix it, good luck.