Why does my water heater stop heating when some outlets don't have power?
Reading your description of the problem, the first thing I think of is a dropped phase or possibly a neutral. So I would start by checking out the panelboard. Look for breakers that are turned on and not reading any voltage on the load side, including the 2 pole breakers and the main.

It may be a faulty breaker, a failure at the meter, or a utility problem. Since you admit you don't have much electrical experience, you may be uncomfortable about opening and inspecting a panel. You will also need to have at least a voltmeter. So I would suggest you bring in someone with experience to help troubleshoot your problem. you can also have the utility company check out there side of the meter.

The most important thing is to stay safe.