Do not Panic!
You have an alarm, now what?
If you have an alarm on your system don't panic. Your system was designed with 48 hours of reserve capacity. It is best to still conserve water. DO NOT take baths, stick to showers, no multiple loads of laundry, hand wash your dishes.
You have back up but no alarm?
If you have a backup and no alarm, what do you do? You need to first make sure there is power to the system. You can check the main breaker box to the house. Another was is to try the alarm test switch on the box. If the alarm does not sound you may not have power. If you do have power and a backup, the most probable cause is going to be a clog. This clog is usually between the house and the first tank. If you are comfortable you can try a plumbers snake to remove the clog or call a plumber.
BACK UP = NO WATER USE
MOST IMPORTANT THING TO REMEMBER IS, IF YOU ARE BACKING UP DO NOT USE ANY WATER. CONTINUING TO USE WATER WILL CAUSE THE HOUSE TO FLOOD WITH SEPTIC WATER!