Five common possible causes:
1) you have a localized leach field pipe break that needs repair - messy but not a very big job, probably about $250-500 range. If this is the case water will be coming up only one place, not over the entire field.
2) your leach field was built with inadequate cover and has "broken through" to the surface as it gradually loses permeability with age.
3) your leach field pipes are plugged and need to be routed/jetted out - typically $400-800 range.
4) your leach field has reached its life and needs replacement - typically in the 4 to 10 thousand $ range but occasionally more depending on local site conditions, soil type, nearest water well, available land and slope, etc.
5) Water table has risen (heavy recent rains or nearby waterway in flood ?) so sewage canot flow down into the ground like it should.
Two approaches -
1) have a septic/leach field construction contractor dig up the spot (if only coming up one specific place) and inspect for broken pipe and fix if that is problem, or if not that then get leach field cleaned (jetted or rejuvenated are terms used by some contractors). If broken pipe that can be fixed, then good to go from there - though you might want to spread a dirt cover over the field if this was not in lawn or grassy area where it can biodegrade rapidly, to cover the sewage. Jetting/surging is an iffy thing - sometimes works quite well if blockage is from buildup of grease and biomass at the drain pipe outlets, pretty useless if entire soil mass has filled with the sewage biomass and grit or with ingrown tree roots because you let shrubs and trees grow on or near it. Typically a field that is less than 20-30 years old or less can be rejuvenated to go another 5-10 years - one that is over around 30 years commonly needs to be replaced by excavating and hauling off fill then replace with new, or in deep groundwater areas sometimes a large number of auger holes are drilled through the field (avoiding the pipes) and filled with sand to add filet and water capacity; or a totally new leach field built nearby to run the pipes to.
2) have a civil engineer who does septic/leach field evaluation and design inspect it, which typically will involve a site inspection with a few hand-dug shallow pits or small boreholes to check how deep the water table is and how thick the saturated zone is and general field soil type and plugging issue and if a general or specific location problem, then typically back onsite later with a drain field contractor to either do cleaning or to dig a substantial test pit or two for leach field testing (infiltration test), which would determine if the field can be saved or not. Cost about $500 plus or minus for site visit, more if deeper holes needed so has to bring a laborer or backhoe.
Option 1) of course the contractor is likely to push for a new field, option 2) will cost a $500-1000 range up front to determine if field can likely be saved, but if decision is that it is not or if rejuvenation is tried but does not succeed so a new leach field is needed soon anyway, then in the long run will cost more than an immediate replacement - but has potential to avoid those many thousands for the replacement.
Obviously, since I misplaced my thousand-mile-away earth-penetrating glasses some time back, I cannot determine which is best for your specific case. I would advise starting with a site visit by a civil engineer experienced in septic design and rehabilitation and see what develops from that, and then decide then on the rejuvenation, overlay with additional fill (if thin cover if the problem), or new field based on what he says. Be sure to get one experienced with hands-on evaluation (shovel and hand auger inspection), not just a walk around and say you need a septic contractor type - and phrase it right that way. An experienced one will either dig test holes himself or have a technician or laborer/backhoe do that while he is there to perform the inspection and tests.
For who - Search the List under Septic Tank for septic contractors (you will have to filter out the pumpers fromthe field construction/contractor types probably by googling). Civil engineer google for septic system design in your town, then check AL for reviews if any.