My B, N, and spacebar keys are not working. How do I fix it?

Did you spill something on the keyboard? It could have dust etc particles that are preventing full contact or the keyboard itself may have a problem.

Turn it over and very carefully pat the back to dislodge any grit that may be under the keys. Don't whack it full force, just slap it gently. You can also carefully blow it out with canned air. If that doesn't do it then chances are the keyboard has a problem and needs to be replaced. That requires taking it apart and replacing it with a new keyboard which has to be for that specific model. Not a job to do yourself unless you feel confident taking a laptop apart...