Q: Engine shuts off suddenly

On one occasion about 20 days ago, the car was parked while was out for a short while. When I was about to continue driving, I put the key into ignition and turned it but the engine was completely unresponsive. All the lights were working fine. I took the key out and there was a vibrating sound made by the starter relay. After I had taken it out and plugged it back in, the car started fine after several minutes. However, now I have the same issue. The engine just died out in the middle of driving. I wasn't doing anything, like braking or accelerating, and the car wasn't idling either. I did the same thing with the starter relay and turned it back on. I had the starter relay replaced. It is even worse now, as the van turns on and dies out after 15 seconds. All the lights still work in the vehicle. What is happening with it? What should I do?