OK, so I replaced the pressure release valve and maybe I broke the
pump in the process. I have a Lochinvar hot water boiler with a B&G
pump heating radiators.

About 10 to 12 years ago, I lived with a hot water heating system. Loved
it, but time moves on...

As I recall, there was a drain and fill event that I lived through, and then
a few hours after the system restart, one of the 5 pumps wouldn't pump. See
if this has anything like yours:

Between the B&G motor, and the actual pump there was a network of springs
and what not that connected the pump shaft to the motor shaft. Memory going
soft on me, but I seem to recall that after the restart that spring assembly
snapped. Was about a $12.00 replacement part. And maybe 3 hours of cussing
and fuming. Once I learned (figured it out) how it all went together, may
be a 12 or15 minute job. (Right tools, right technology, right training,
.....you do need all three)

As I recall, I just went to a plumbing supply place that also listed Hot
Water Heating Systems parts as part of the store's inventory. Not all
plumbing supply houses carried Hot Water Heating parts. I just described
the spring thing, and they asked about the size of the hot water pipe, and
knew what the part was, and sold me what I needed.

Seems there could have been a large air bubble going thru the pump, and this
spring assembly goes busted. Don't know if by design or safety feature to
save either pump or motor. Generally, in 3D, the shaft of the motor may
not be in the exact alignment with pump shaft, so spring assembly bridges
the two shafts and transmit the power to the pump.

As I said, may not apply to your case, and my memory is not as good as it
once was.

Phil