This has baffled me and plumbers. My neighbour had a basement bathroom built 30 years ago
I replaced the old toilet a year ago to a low flush. Everything has been working fine until the city turned off the water for 2 days to repair a broken main. Now the toilet will not drain. I removed it and there appears to be a p trap below the toilet. I pour water down and it drains. The water level is about 5" down the hole and does not change no matter how much water I pour down the hole. I placed the toilet in the shower and found no obstruction. I poured water in the bowl and it drains fine. I reinstalled the toilet and it would not drain again. How is it that it worked all this time and now there seams to be an air lock between the 2 p traps. I would guess venting but how?

something does not make sense here.
if there truly is a p-trap the toilet would never have flushed irregardless of the city turning water off and back on..

That is what I thought. It has worked fine for 30 years

Auger the toilet. The city turning the water on or off has no bearing on the drain system at all, only the supply.

You say it "baffles plumbers", which implies that they have checked it and it DOES have a trap under the floor. IF so, the ONLY way it might have flushed is if it had a "healthy" amount of water to do the flush and force itself past the second trap.

Not a plumber, but if you pour water down the bowl and it drains, but flushing doesn't, then it seems to suggest what hj said that there isn't enough water when you flush to push the water through the double trap.

May be when the city did its repair debris got into the pipes? Then it made its way into your toilet tank and clogged up the flush valve pathway or rim holes to the point that the flush is slowed below the needed thrust to create the siphon?