<p>Flsun 3D Cube; Marlin 1.1.1; main board: Makerbase MKS Gen_L V1.0; running from either Repetier or OctoPrint.</p>

<p>I was recently obliged to replace the main board when it stopped powering the heated bed. I got the new main board - same make, version, etc - got everything setup just as it was before, but the bed still doesn't heat. Multimeter shows zero across the board's heat bed contacts, whether using G-code from the terminal (in both Repetier and Octoprint), G-code in the print file, or the control panel on the front of the printer. The thermistor works: if I shine a heat lamp on the bed, it registers the temp change.</p>

<p>Bad board? Something in the Merlin config I missed? Is the board smart enough to not power it on if the bed heater itself is bad?</p>

Considering:
Multimeter shows zero across the board's heat bed contacts
this implies that
If you measured
resistance
, the heated bed has no resistance. Basically this implies that the bed has a short. This might be the reason why it is not working. If you would power it as such you create a short. Instead of replacing the board, you need to replace the heated bed. Typical values for a heated bed of about 200 x 200 mm are in the order of 1.2 Ω (measurements between 0.9 and 1.5 Ω are reasonable to be expected).
If you measured
voltage
, the heated bed does not receive power for heating, or the power does not reach the bed (not turned on or broken wire?). It would then be wise to measure the resistance (of the bed and the wires). If the resistance is in the order of about 1.2 Ω (see above) for the bed, you could try to connect the heated bed directly to the PSU to see if it gets warm, if so, please disconnect immediately to prevent damage. From this experiment you can find whether the heated bed is broken (or the cables), or that the board is not functioning correctly, this is, however, strange as you tried 2 boards. A possible suspect could be the MOSFET that schedules the powering of the heated bed if you use an external MOSFET board that it.
If you measured
current
, then you found out that no power is delivered to the board, but you also might have broken your board in the process, as measuring current is a (close to) 0 Ω connection and has to be done
in line
of a circuit.