Without the water in the U-bend - there is a direct connection to the soil pipe and foul water drain (sewer) - hence the smell !
Water in the toilet bowl prevents the gases and smells from the pipe escaping into the house - these are vented via a 'stench pipe', which you may see outside rising from the soil pipe up to roof level - often with a small mesh covering on the top.
I would guess, as Top_Gooner has said, that as the toilet appears to be unused, the water in the bowl has evaporated. Check that the bowl itself isn't damaged or cracked (unlikely), and re-fill the bowl by flushing. Even if the cistern did not work - you could just fill the bowl with water from a bucket, enough to fill the U-bend and prevent nasty smells coming up from the drains.