low oil
I suspect that you've had an oil leak for some time. Did you recently change the oil filter? When the oil level dropped far enough the hydraulic lash adjusters became starved for oil and began to make noise. Eventually as the oil level continued to drop there was not enough for the oil pump to maintain pressure in the engine. The oil pressure gauge is driven by an oil pressure switch instead of an oil pressure sensor. It works the same as an idiot light. I don't have the specifications on the pressure switch but I estimate it is activated around 10 or 15 psi. If you shut the engine down immediately when the "gauge" went to zero then the engine bearings may not be destroyed. If you add oil and the "oil pressure gauge" goes back to normal than I suspect your oil pump is still working.
I suggest that you drain the existing oil from the pan and replace the oil filter because they may contain minute metal particles that can cause further damage. Filling your new oil filter with oil before installation reduces the length of time the engine runs without oil at startup. If the engine starts and runs quietly after a few minutes then you were lucky. Check for oil leaks at the filter, the oil pressure switch, the drain plug, the oil pan gasket and the rear main seal. Check your oil level daily until you find the cause of the leak and correct it.
