Put a multimeter into both pins of D8, turn on the bed in software, do you get 12v?
Also try the heated bed with the bed +12v plugged into D8 + but the gnd directly to gnd on the power supply.. This is un controlled heating, but lets you test the entire 11amp cct, threw screw connectors, and fuse, but bypasses the mosfet (on the gnd wire)
also remove the ramps, load up the led flashing demo sketch and change it to pin 8, check you get 5v on D8 when its on.. this tests the mega.
but its most likely the mosfet is dead. they are not very well suited for this job and are easily damaged by static
replace with a IRLB 8743
Also try the heated bed with the bed +12v plugged into D8 + but the gnd directly to gnd on the power supply.. This is un controlled heating, but lets you test the entire 11amp cct, threw screw connectors, and fuse, but bypasses the mosfet (on the gnd wire)
also remove the ramps, load up the led flashing demo sketch and change it to pin 8, check you get 5v on D8 when its on.. this tests the mega.
but its most likely the mosfet is dead. they are not very well suited for this job and are easily damaged by static
replace with a IRLB 8743