Put in new spark plug if the one in there is a bit old. It may spark outside the engine, but inside may fail. I had that with my KLX650, wouldn't start unless bumped off. The plug electrodes were worn enough they wouldn't spark under the compression and all in the cylinder, but would spark like normal outside. Replaced plug and bingo!

Other question - has gas in tank, has gas in carb or has signs of gas mix in cylinder? Obviously if there isn't gas in the tank or not getting to the carb it won't start, but just asking to be sure what you mean by has gas. If your choke and pilot jet are plugged it will not start, You might bump it off, but it may stall at idle. Both signs you have crud in the choke and/or pilot sytem. Needs cleaned, probably best done by some shop or person with an ultrasonic cleaner or a carb washer.

Hope that helps.