Sounds like the motor has gone bad, at least. Probably needs to be replaced, depending on the age of the house and fixture.

Would the motor being bad explain the lack of voltage at the switch though ? Like it’s shorting in the motor or something ? It is very old fixture.

I have limited experience, but depending on the fixture there can be (note can be) a smaller fuse inside the motor that could interrupt the flow. Might find a part number and see if there's a manual running around the internet.

Are you sure you've got the voltmeter set on AC and not DC?

Your switch should be reading 120, coming in when it is off. If it's not, then you have an issue prior to the switch.
Without seeing how your switch box is set up, it would be hard to tell you where and how to proceed right. It depends on where the power is feeding first.
If you can post some pics, that would help too.

If it is a bathroom fan with a heater, check to see if there is a breaker for it that may have tripped? These can draw a lot of power and often have a dedicated breaker.

You need both a switch and a fan motor.

Need to trace back and find where there is 120v and then troubleshoot from there out.

It could be a switch, the unit itself, or even a bad breaker if the heater is high load and the house is older breakers do go bad.

You should have power at the switch at least, no matter whether the device is broken or not.

This leads me to believe the problem is upstream of the switch. If your switch doesn't have a neutral (only hot in, switched hot out) it likely gets power from the fan's electrical box.

Since you said the light fixture still works (combined in the fan?) your problem likely is located behind the fan or wherever it gets its power from.

Could be a loose wire nut, or a shorted/melted connection within the fixture. Take it down and inspect all connections.

Since you said the fan doesn't work even when connected to working power, that means it's definitely gone

Yes

If it is set on AC, I agree with other posts about something upstream like a tripped breaker. There should be 120V at the switch.

I ask because I often go between testing AC and DC and sometimes forget that I have it set to DC when testing and AC circuit, and vice versa. That low 0.3V is often what reminds me to change it.

So behind the switch there is 2 incoming sets of wire, the one that goes to the lights, and the one that goes to the heater and fan. No breakers are flipped at all. Is there a way to follow the wire with no power back to the breaker ? (To check for a bad breaker possibly) or could I cap off the other set and use the wires from the lights to power everything if I replace the unit ?

4 wires just connected to two switches? Then it gets power from the fixture, not at the switch box

Okay so basically it would probably be that the one set goes in to the fixture and then it splits there and comes back to the switch ?

Sounds likely if you only have four conductors plus ground inside the switch box.

That makes so much sense. Ill try getting more of the fixture apart soon. I usually only work on 12v electrical and it seemed like something was shorted in line to me. Thank you for the advice.

Is the fan maybe humidity controlled? Any small dials / knobs that are visible on it's back? We had such a bathroom fan, and the humidity detector was faulty, but luckily, twisting the "sensitivity dial" to max, just kept the fan on continuously.

Other than that, does it spin if you just push it? As in; is it able to rotate freely?

Might also be nice to know what make and model it is if that's available =)

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One thing that stood out, was that you measured only .3 volts at the switch? as in 300mV?.. That's absolutely nothing, so either there is no available power there, or something on that line is short-circuited (which would drop the potential to nearly 0 volt) but that would also cause any fuses/circuit breakers to pop.