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!
Try right clicking the game in your library and hitting properties. Then go to the Local Files tab and check the disk usage. You likely do have data downloaded and there is probably an update required.
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.