What now? Provide enough information.
Actual LR Version Number?
How are you connecting to your images?
What actually happens? Error Message? Text of message?
For all we can tell, you might have forgotten to plug your card into a reader...
Like Rikk said, the latest version of Lr should import those D750 raws. So if it isn't, you need to put on your Sherlock Holmes hat and start trouble-shooting.
That said, one common reason you can't import Nikon files that should be importable is they've been molested by Nikon Transfer, or older version of Nikon View.., more info here:
If you want, you can post a link to one of your D750 files so others can try to import (or conversely, download a known importable D750 file and see if you can import it).