Marion Greene, if the sewage is not surfacing above the septic tank, nor the leach field, then the problem may be a clog in the sewer pipe system in the house, or in the sewer pipe from the house to the septic tank, or perhaps there is a clog in the outlet tee of the tank.
Expose both lids, (inlet end and outlet end) of the septic tank. If the sewage level is up to the outlet invert elevation (over flow elevation)of the outlet pipe, then the clog is in the sewer pipe. If the sewage is higher than the outlet pipe of the tank, then the leach field is failing, or there is a clogged outlet tee.
If you determine that the sewer pipe is clogged, call a Licensed Master Plumber to water jet the sewer pipes to scour the accumulated grease, hair, soap scum, slimes, and other organic matter inside the pipe.
If the leach field is failing, contact the local septic tank regulatory agency for advice, a permit, and a recommendation for a local Licensed Septic System Contractor to repair the system. Good Luck! JWA