Unless there is something untoward I would presume the water is just turned off at the stopcock to prevent flooding risk while the house is unattended. Anyone can turn it back on.
The percolation area is the area adjacent to the septic tank where treated wastewater soaks away into the ground. Solid waste remains in the tank, to be eventually desludged. If the tank is not maintained there is a possibility of solid waste overflowing between compartments of the tank and entering the slotted pipe which goes to the percolation area, contaminating it and stopping the whole system working. In the worst possible case the whole tank could have to be dug out and replaced, an extremely expensive job. With respect -- it sounds like you know very little about septic tanks. You really need an inspection by a qualified person. There's no way anyone here can guess as to the state of a septic tank. But as I said, there's no a priori reason to suspect it's in trouble just because it hasn't been emptied in quite a few years.