3 main things necessary for combustion: spark (which you said you have), compression (which you probably have if it was running), and fuel, which seems to be your problem.
The PRI (prime) position on your petcock lets the fuel go directly to the carbs, any other position requires vacuum to allow fuel out of the petcock. I suspect a vacuum leak in one of the lines. I would look for a split in the end of a vacuum line, check them all. Did you fiddle with anything just before it quit running? If you did I would look there first.
Don't worry about "not being good with bikes", you are getting experience as the hours roll on. Experience is something you get right after you need it. And good judgment comes from bad judgment. Just embrace this event as educational experience.