No power in half of the house is regular service call request. Two of the more common causes includes, the a failure on the electric power companys' line to your property or a breaker malfunction.
Okay here is a quick first lesson in electricity. The average home has a 120/240 volt main electric service. There are 3 wires from the electric power company to your home, 2 live wires and 1 neutral wire. The 2 live wires we will name phase "A" and Phase "B". Phase "A" to the neutral is 120 volts. Phase "B" to the neutral is 120 volts also. The two 120 volt wires powers' all your general lights and receptacles around the house. So if one phase wire stops working half of the house will have no power and the electrician will call it “losing a phase”.
Lesson 2 is about 240 volt part of your electrical system. Phase "A" to Phase "B" is 240 volts. The 240 volts system normally powers things such as your electric stove, hot water heater, clothes dyer, air conditioner and furnace. So if you “lose a phase” these appliances will not work properly since they may only have 120 volts being supplied to them.
You have about a 50/50 shot between calling the power company or an electrician regarding your power outage. If it’s the power company wire problem they may fix it for free. If it’s not a utility company issue then you will have to call the electrician. Causes for this may include a faulty main breaker or a loose connection in the meter can, main breaker, main electric buss or a bad feeder wire. Loose connections resulting in overheated parts will require replacement of the faulty part(s), wire(s) or the entire electric box itself. Unfortunately, all these repairs are expensive.
Sometime a single circuit breaker or fuse being off will be mistaken for half the power is out. So check your main electric box for a tripped breaker of blown fuse first. I don't like paying an electrician $200 or more, just to reset a circuit breaker.
Contact your local electrical contractor or electrician for more information.