Had headlights, then high beam only, now nothing

Don't trust hi current stuff with an ohmeter. Under load, the contacts can fail

Just trace it out, you have a diagram?

Functional path is from HL switch to dimmer, out the dimmer on low and high beam wires, through the bulkhead, and to the headlights, which are grounded through a common ground. So check that

Backtrack with your meter/ test lamp. Figure out where the HL are in the bulkhead connector, see if you have power there, backtrack to the dimmer