Most Exmark zero turn lawn mowers have three safety devices in place in order to get the mower to start or run. If one of these safety devices are not engaged then the mower will not start or run.
In order to get the lawn mower to start you have to have the brakes set, the blades disengaged, and be sitting on the seat. In order to keep the lawn mower running you have to be in the seat. The only way to keep the mower running and get out of the seat it to have the brakes set and the blades disengaged.
It sounds like the safety switch below the seat may be bad or disconnected. If the switch is bad it would need to be replaced.
The model and serial numbers on your Exmark zero-turn mower can be found on the frame, under the seat.
NOTE: if you need a part for the small engine in your Exmark zero-turn mower, there will be a separate model/serial number for the engine. Here are the small engines that are available in your Exmark mower. Visit the page of the engine you have to find out where the model/serial number is located for your small engine model: