Heated Bed not heating on original Prusa i3. Using a Ramps 1.4 EFB
First you should check if the bed still works when applying 12V directly. You can then also check the voltage over the 12V output terminals to measure the power supply unit while it is powering the bed. It is unclear whether you measured the voltage of the PSU while it was powering a large load.

It is also a good idea to get the external MOSFET board out of the equation (the less components you have, the less you need to troubleshoot).

From the post can be concluded that you are using the so-called RAMPS EFB setup, e.g. in Marlin firmware defined by constant IS_RAMPS_EFB in your firmware printer setup (Extruder on pin 10, fan on pin 9 and bed on pin 8).

In the pin configuration file pins_RAMPS.h you can find the following lines:

#define FAN_PIN        RAMPS_D9_PIN
#define HEATER_BED_PIN RAMPS_D8_PIN


To change the pin layout, switch the values of the constants.