Well there are a few things that could go wrong.
- The thermistor has died: simple enough to check, if you know what type is you can get the datasheet and use a multimeter to test it or B place a cup of boiling water on top of the sensor and see if the temp rises in the controller interface.
- Power supply failure: don’t know what setup you are using but it is possible for mixed voltage setups again simple enough to test with a multimeter. I’ll assume your using a ramps controller and a single 12v supply if everything else still works then it’s not the supply.
- The bed has failed: the heating element on the bed may have failed, to test use a multimeter, test for voltage at the bed side of the cable, if there is no voltage you may have a break in your cable or … see next.
- The transistor has blown on the controller board: this can happen quite easily, it’s a bit harder to check if you don’t know a little about electronics it will also depend on your controller board… some have a led that blinks coming from the transistor if it’s not on then it could be a blown transistor.