This problem can have a number of causes. Is there fresh gas and oil? Is the fuel shut-off valve turned on? Is the on/off switch in the on position? Most often, the spark plug is damaged or malfunctioning and has to be replaced. It is also possible for the issue to be due to fuel line damage. The carburetor could be blocked or damaged. There may be compression issues.
The problem may come from the starter itself. The battery of an electric start can get drained out. Were there electrical devices plugged into the generator prior to starting the unit? Generators need to stabilize and warm up before appliances can be applied. If the generator still does not start, it is best for the device to be checked by a Generac Service Dealer who will diagnose the problem and provide resolution.