Heated bed problem

[printbus]

The fuses on RAMPS are polyfuses - they automatically recover after an overcurrent situation has been detected. These are the large and thin yellowish square things next to the power input connections. So, we should be able to rule out a fuse on RAMPS being burned open.

Let's divide-and-conquer. Assuming your printer doesn't use an off-board relay to power the bed, the bed should be wired to the D8 output on RAMPS. The D8 output is controlled by the 3-pin MOSFET switch right beside the D8 and D9 screw terminals. The bed MOSFET switch usually has a U-shaped heatsink on it. RAMPS has an indicator, LED2, wired to the D8 heat bed output as well. LED2 is on the board, likely inside the U formed by the heatsink. Does LED2 toggle on and off as you attempt to turn the heater on and off?

If LED2 is not toggling, there's likely a problem with 12V not being applied to the 11A input on RAMPS, your firmware thinking there's a problem reading the bed temperature (e.g., bed thermistor or wiring to it is open or shorted), or a hardware failure on RAMPS.

If LED2 is toggling, your problem is downstream from RAMPS, and you have an open in the wiring to the heat bed or the heat bed itself has failed.

To help further, fill us in on important details as others have suggested. Info on how you are controlling the printer (some particular host software vs. local control panel), what firmware the printer is running, whether you have a voltmeter, size and type of the heat bed, etc. are all important to understanding what you have so that diagnostic help can be provided.