There are several causes for a foul sewer smell in your home or basement:
Dried out P-traps. Water in any trap under unused drains will eventually evaporate, allowing sewer gas to come up through the drain. Pour a pitcher of water followed by a couple of tablespoons of cooking oil down the drain. This restores the trap water, and the floating seal of oil will keep the water from evaporating.
A broken seal around the toilet in the wax ring or caulk. If the wax ring is broken, you’ll need to remove the toilet and replace the wax ring. If there is a crack in the caulk, apply a bead or line of caulk around the exposed area.
Cleanout plug issues. Remove the grate that covers the opening and make sure there’s a plug inside the drain bowl. If the plug is missing, there’s a direct path for sewer gas to bypass the water trap. Sometimes, the plugs are removed to clean sewer lines and not replaced. Buy a replacement plug at a hardware store.
While unlikely, the water in the toilet trap also could have evaporated. Weeks of no use could cause this. Flushing will restore that water.
Broken or cracked sewer pipe. You’ll need to call a plumber to do a video inspection to determine the severity of the problem.