Bed Heating Failed, Printer Halted, Please Reset
Check that your heated bed is still working: 


Measure the resistance of your heated bed. It depends an the power
rating of the bed, but 12V beds usually have values &lt;3 Ohms. That's
hard to measure for cheap multimeters, but you just need to check
that it's not MOhms which would indicate a broken heated bed.
Your heated bed could also have a short. That's hard to measure, as &lt;3Ohms are already 'almost a short'. You'll only notice because your
bed will not get warm, but the wires, connectors and elements on the
RAMPS will get even hotter. Marlin should detect this and switch off
after a few seconds.
Check your thermistor. Seems you already did that.
Check your RAMPS by measuring the voltage on the bed connector while setting the bed to heat up. You should see 12V, at least in the
frist few seconds.