So I downloaded the first 12 gigs of NBA2K15 on steam about a week ago. Then yesterday I downloaded the rest. When I finished the download it came up as I had to download it again, saying "update required" I noticed when I first downloaded the game my space on my c: did not fill up which it should have? I don't have enough internet to download the game again. Help Please!
I would assume there is some sort of data corruption going on here which unfortunately make the data you downloaded useless to you.
If a restart does not correct the issue i am afraid you will need to download the files again. If you do not have enough data allowance left you will have to wait, or try tether via your phone/dongle.