CHECK (OR HAVE US CHECK) THE WATER LEVEL IN THE TANK

If the water level inside the tank is lower than the outlet pipe, chances are you have a clogged or broken pipe between the septic system and your house. This is a pretty common repair that can sometimes be as simple as calling a plumber.
If the water level inside the tank is higher than the outlet pipe, the problem generally lies with the tank itself or part of the leaching field. This can sometimes be as simple as a clogged screen. Generally, you'll want us to pump out the tank. This will allow us to see if there is a simple solution and give you the ability to continue to use toilets, sinks and showers while we figure out what's going on.
SOGGY SOIL BETWEEN HOUSE AND TANK

Soppy soil between the house and your septic tank is generally a pretty good sign of a broken pipe. Sometimes this can be tree roots or simply decaying pipes, or perhaps you were digging in the area. While not as simple as a pump-out, this is generally a fairly straight forward fix.
AEROBIC TREATMENT UNITS

Aerobic Treatment Units aerate the tank to help break down waste faster. If you've been away for a long period of time, the good bacteria that helps your tank work may have simply died off. Generally, using your tank sparingly for a couple weeks will help the bacteria grow back to usable levels.
LEACHING & DRAINAGE FIELD ISSUES

Unfortunately, drainage/leaching field failure is not generally a "fixable" problem. Most often, failure of the tank from not being pumped out often enough causes semi-solid waste to accumulate and clog up your leaching/drainage field. When this happens, your field can no longer drain effectively and will need to be replaced in part or in full.
The good news is there are multiple directions you can go here (with varying costs) and salvaging the current tank will often save you a significant amount.