1st, check the fuse box, read on the label and then test for a current. if has a current and the fuse is good, go to step 2
2nd, check under your seat where the electric seat belt motor is, test it to see if there is a current. If not re-trace steps.
Last but not least if all else fails and you have failed as a handy ameture mechanic, take it to a nissan dealership n the seat belts have a warranty i hear, once again hearsay. It doesnt hurt to call and ASK. My personal advice do it your self so you dont have to rely on other ppl and it cheap. lol