Bad Smell in the Basement
The main cause for a bad smell in the basement is the floor drain has dried out and now sewer gas is venting into the house. The floor drain in your basement is similar to the drain in your tub or shower. It is a U shaped drain. The U shape holds a small amount of water that acts as a seal in keeping out sewer gasses. If the drain dries out, there is nothing from stopping the gasses from being vented back into the house. The solution to this problem is very simple. Pour about a gallon of water into the drain. This will refill the U shaped trap and stop the smelly gasses from coming back up. Check the following day, if there is no water in the trap that probably means that your trap has a leak and need to be repaired or replaced. That will involve digging up part of your basement.
Any plumbing fixture can dry out due to evaporation from lack of use. To fix it, try some water first and save yourself a plumbing bill.