Small engine

I have a 3500 watt Briggs & Stratton generator that is not even a few days old. I assembled the unit and filled it with the oil that it came with. I ran it a couple nights for about 5 hours at a time. The third night I started it and it shut off. I checked the oil level and there was none in it. I filled it and tried to start it and I cannot even get the pulley to move. Is the engine seized up? When it shut down it didn’t make an irregular noise like as if you could hear metal breaking it just shut off like if you had flipped the switch off.