If you didn't mix oil with the gas it won't run "for awhile" but it run for a short while...

If the mixture was a little lite or it was running lean then it can run for a longer time. Without looking at it or knowing what brand/model it is, it's hard to tell you exactly what is wrong.
Does it have spark? If it does will it run if you shoot a little carb cleaner in the carb? If it will run for a few seconds it's a fueling problem, could be the carb is dirty, the fuel filter fell off the fuel line, a cracked fuel line or primer. Also check for loose carb or cylinder bolts (try to move them by hand). One other thing to look for is a plugged muffler.
If there is no spark most likely your ign. coil went bad. But also check that the plug didn't get some carbon on it and closed the gap.
I start basic trouble shooting by pulling the pull rope. Does it have compression? This tells you the general condition of the piston/cylinder. If you can't pull it over check for a bad shaft, blown clutch (if so equipped), trim line wrapped around the shaft (happens more often then you think) or a seized piston.
Next check for spark and the condition of the plug. No spark most likely means a bad coil but could also be the cutoff switch ar even a bad plug.
See if it will fire with carb cleaner shot in the carb. If it will fire, it's fuel delivery problem. Check for dirty carb, filter/fuel line condition.
If it has spark and won't fire with carb cleaner then look for loose cylinder bolts, loose crankcase bolts or loose carb bolts... most likely some type of major air leak.