The oil may have drained slowly through the oil pump and is now in the sump.
The first thing to do would be to remove the sump plug and collect any oil that may be in the sump (in a clean container) and tip it back into the oil tank, not forgetting to replace the sump plug after draining the oil from it.
Before starting the engine, momentarily disconnect the oil feed pipe at the engine end to ensure you have oil present at this point.
It should then be OK to attempt to start the engine
