Why does my power keep going out to most of my house?
There are some simple things you can do and some things you should be aware of. At least if you do call an electrician (which I recommend you do), you can be educated. After all, this is a DIY Q&amp;A site. :) That said, messing around with electrical systems in your house can kill you and your family. This is just, what I would do bearing in mind that I have some experience and you may / may not. I'm not responsible if you die, please use caution and only do things you're comfortable with. However, calling your power provider is never dangerous, I would do that next:

Call your provider and ask them what service level you have at your house. Do you have 60amp, 100amp, 200amp? etc. This is important. If you have 100amp or less, that's probably at least one of the issues if not the entire issue. It's common in old houses to have only 100amp or less. Anything built in the last 10 - 15 years will likely have at least 150amp or 200amp. This is important because if you have 60amp service for instance, even in a 2 - 3 bedroom house with average appliances, it's likely you're throwing the main breaker because you're asking for more than 60amps. An AC unit for instance might take up half that on it's own. Add a freezer, then a microwave, and a home entertainment system, you're already at 60amp.

So, first thing I would do is find out your service level. Bear in mind, this can be upgraded. You'll want to pay an electrician to do it. I had this done 2 years ago, and it was $2200 to upgrade from 100amp to 200amp with a new GE breaker panel included.

You said the main breaker is getting tripped. And you said it won't come back on. As others have hinted, it sounds as though there is an appliance coming on at certain times and tripping it. Next time this happens and you can't get it to come back on, try turning off all the breakers. Then, turn on the main (it should stay on when everything else is off). Then turn on one breaker at a time until you get to the one that trips the main. This way you can figure out which circuit is causing the trip. Before you do any of this, I would take a visual of the breakers. Make sure nothing is overheating, looks melted etc. If something doesn't look right, I would have an electrician out immediately. It's not worth catching your house on fire.