Running a generator with low oil can permanently damage the engine. Place the generator on a level surface. Remove the oil filler cap/dipstick and wipe it clean.
If you have the in hand, you turn to the "Troubleshooting" matrix. Here is the step-by-step logic it provides: honda ex500 generator manual
If your EX500 fails to run properly, use this logical diagnostic sequence before seeking professional service. Engine Will Not Start Running a generator with low oil can permanently