My Ninja 250 won't start. Help! Fixed!

I have had my Ninja for just over a month and it has always started with no problems. Last week I rode it on Saturday and then didn’t use it till Tuesday morning when it won’t start.

It did spend Sunday in the rain when previously it was always put in the garage, but surely one day in the rain wouldn’t do this? Has only one else had this problem?

I have checked that its not the kill switch, tried starting it with the stand up and yes there is petrol in the tank.

Any suggestions?

UPDATE: I went home armed with instructions on how to remove the ECU. When I got to the bike I thought lets try one last thing MF had suggested (that there might be a vacuum in the petrol tank) so I opened up the petrol lid, closed it and then tried to start it and 5 seconds later, it started! I still cannot believe it was something so simple. I went for a ride on Saturday and she ran fine. This morning she started first time so I thinks its problem solved.