+1 This is often the cure for this type of situation.

Remember - simplest solution first.

 

faulty Stop/Run switch.

 

That's not a good thing ,, ask yourself this ,, is there a gas puddle under the bike? Where is the leak coming from? Fuel line? Carb overflow / flood tube?
With the petcock off ,, dis-connect the fuel line .. try to start it ,, it will start , or it should , as the fuel level gets lower in the float bowl ,,then it will die out . Then get some seafoam and a small funnel that will allow you to fit it at the end of the fuel line,, put just enough seafoam in to fill up the line,,start it briefly 5-10 seconds ,,then cut it off ,,refill the line and reconnect the fuel line with the petcock off ..let it sit for about 30 mins ,,then turn the petcock on and fire it up and see if the seafoam cleared up the problem ,,I.E. sticky float valve /seat . This has worked many times with old bikes I've picked up that has been sitting for a long while ,, and just a week ago on my son's dirt-bike.
Check to see if this is not a fuel line leak 1st . The faulty stop/run switch that adrenalnjunky pointed out is a very valid cause too .. disconnect the kill switch and try to start it ,,if it starts,,there's your problem.
That has happened to me 2 times with a bad kill / run switch / wire grounding out from the switch. BTW ,, you never answered if you got it started with the fuel cap off / fuel door open .

 

No it didn't start with the fuel tank open... Checkin the faulty switch tomorrow. ... Thanks

 

I've filled up and drove straight home. Morning after Bike wouldn't start until I opened the gas cap then started.

Any One have an explanation as to why this happens?

 

possibly a plugged vent line in the tank?