You need to check to see if it has spark, if it does then you check if its getting fuel, easiest way is to put a rag over the spark plug hole and turn the motor over a bunch of times, then check to see if the reg is moist or at least smells like fuel. If you have both of these and it still wont start, it is either a problem with engine timing and, or lack of compression. You might just try a new plug and gas i've had bikes with the same symptoms as yours and changing those made a whole world of difference