Aquastar 125x flame stayed on--now low flow & won't trigger

[kbanshee]

Our Bosch Aquastar 125x has been perfect for 25 years and then the microswitch died a month ago. No plumber would look at it. So I replaced the microswitch. When I replaced it, a leak started. I tracked down and replaced the water valve. Running fine for several days. Today my spouse showered and I wasn't near the water heater when he finished...it for some reason didn't turn off the flame when the water turned off. We didn't notice for several minutes til pipe banged & I went to check why. We were on way to doctor so I shut it off. On return now, we have little water flowing thru (flow if turn on hot water tap is trickle...flow if turn on cold water tap is normal). It is not triggering the pilot or heating water. Where do I look for what issue the (probably over-) no water flow/heating likely caused?
Thanks
Kriss

 

[Fitter30]

If the flame didn't shut off and there wasn't any flow the heat exchanger is twisted up and very surprised it's not leaking. Any tube style heat exchanger has to have flow to get rid of the heat because they hold very little water. 25 years old your way a head of most by at least 10 years. Must have soft water and had yearly maintenance. Replace it.