I called Apple and got a solution that might help. If it’s a software issue and not a physical issue (like a cord), then turn off the MacBook, press the power button, then right away press and hold “command” and “r” until it turns on. Worked like a charm, and then all you have to do is click on the Apple logo and do a software update. I hope this helps, I was beginning to really panic since I had just bought a used 2015 MacBook and didn’t want to have to contact the person I bought it from!!