Septic problem

OK. Here’s the situation. We have a small unit with a 500 gal septic tank. Has worked normally until the last 2 weeks. Developed a backup which came up through floor drain. First time it was clear, but next time was “dirty”. After a few minutes, it goes down–slowly. Tank has not been pumped for 6 years. Only one person lives in the unit. It’s been VERY cold here the last month or so-VERY cold. Like often sub-zero. I am thinking : 1. tank is too full of solids and needs pumping or 2. leach field may be too close to the surface and has partially frozen, thus blocking flow to dispurse water. I am really hoping that the leach field has not failed- but I see no evidence of that-no water at surface and no “sewage” or smell in the area of leach field.

Any plumbing experts out there who might give me an idea or a rundown of a septic system troubleshooting guide and some possible cost ranges? I have a current quote for $120 to pump the tank (I think very reasonable). That would be my first thought, but if it doesn’t cure the situation, then what?

Thanks for the help.