Tenant's heat not working after gas line switch over

I take care of an old (1850s) apartment building. The utility company has been installing new gas mains on the street and on Monday the changed over the connection for the building. One of the units, the bottom one (its three apartments in a brownstone) has no heat. The burner is working fine, pilot is on and hot water is going up the pipe. I have bled most of the radiators (about to do the last one) and have not gotten any air, just water. The pipe leading out of the heater is working fine, but not a single radiator pipe is even warm. I assumed it was an air blockage, but there is not air coming out of any radiator. What other things could be stopping the hot water from flowing? Heat worked perfectly fine until the gas line switch. I assumed air got in the system but none of is venting out.

What else should I try here? I am confused.