heatbed thermistor input not working (no replies)

Hi All,

My heatbed on my RAMPS 1.4 stopped working last night (mid print), and I'm trying to track down why, Symptoms are this:

12V present on both inputs, and I can see 12V between all the D8/9/10 + pins and ground
I've got a known good thermistor and can plug it into the extruder thermistor input and see correct temperature values being reported for the extruder
When I switch that same thermistor onto the extruder bed pins it's reports zero degrees, which generally means nothing is connected to that thermistor input (correct?)
(I've got the same thermistor type (#5) set in Marlin for both inputs)
I can see 5V on the termistor input pins, so it looks like it should get a voltage back?

It appears that the thermistor input pins for the heatbed aren't reporting correctly, so the firmware won't power up the bed (assuming no thermistor is connected).

Any ideas on what to look for? I'm pretty sure something on the ramps hardware side of things has died, as it just stopped working mid-print. Repetier Host showed it was still supposed to be on, and everything else kept going quite happily.

Thanks,