DIAGNOSIS If both the top and bottom heating elements are completely dead while the electronics work, the most likely culprit is the main thermal fuse (a safety cutoff) has blown. It's a one-time fuse that trips if the oven overheats.
FIX This requires disassembly to access the rear panel. You'll need to locate and test the thermal fuse, usually a small white ceramic component with two wires, near the convection fan housing. If it has no continuity, it needs to be replaced with an identical-rated fuse.
IF THAT DIDN'T WORK The relay on the main control board that sends power to the elements could be faulty. You'd hear a distinct click from the board when a heat function starts; no click might mean a bad relay. That means a board replacement.
HEADS UP A blown thermal fuse is a symptom. It often blows because the convection fan failed or was blocked, causing overheating. So if you replace the fuse, you MUST verify the fan is running smoothly, or you'll just blow the new fuse.