Sometimes, after years of buildup of hair, garbage and other gunk that goes down your drains, all the pipes in the house become backed up. The culprit is most likely your main drain going out of the house, or the drain between the house and the sewer (or your septic tank) being clogged.
If you have a septic tank, you'll want to clean it out. If you have a main drain in the house to the sewer, clean that out.
To clean the main line, there is usually a cleanout at the farthest end of the drain, a plug that lets you feed an auger in without going through any traps. You find this in the basement or crawlspace. This would be work for a power auger, which can be rented for the job.
Electric augers are big with a lot of cable. If augering the main drain in the house doesn't help, then augering the drain from the house to the outside would be next. There is usually a cleanout just below the point where the main drain leaves the house. Remove the cap and start augering again. When using a power auger you won't feel resistance when you get to the clog. You will hear the motor start to slow, and that is when you need to reverse the motor and back out, going back and forth till you cut through the clog. Be prepared, this is a messy, smelly job.
If you have a septic tank, you can't tell if the drain is clogged or if you need the tank pumped out, but the smart bet is the tank will need cleaning first. If you have a drain going out to a municipal sewer, you may want to check with neighbors to see if anyone else is having trouble, it could be in the city's drains at the street.