A couple of easy possibilities come to mind. First, if you haven't primed it (by holding the switch in the off position before starting it), try that. Many or most Onan generators will run the fuel pump without cranking the engine to get fuel from the tank to the engine. It can take 30 seconds or more, depending on how long the fuel line is.
Second, check that the oil level is full.
Beyond these easy things, it's pretty much a matter of basic small engine troubleshooting. Do you have fuel? If no fuel, do you get fuel when you try to run it taking fuel from a can with a short piece of gas line? Do you have spark? Is the carburetor clean and the choke mechanism operating? etc.