Some ideas... (DMM required)
Given that it happened just out of the blue, I'm unsure. I'm going to hope you have a Digital Multimeter and know how to use it. I can think of a few possibilities for what might be the problem. It sounds like a big, clean break somewhere:
First off, check for continuity on the actual plug by touching one probe to the sleeve, and one to the tip (if there's a normal impedance, life is good.) Then measure from the sleeve to the middle ring (again, check for normal impedance.) If one of these leaves the DMM reading "1" or "OL" (something like that) then there's no continuity, and there's a break somewhere. If that's the case, then:
A. There's a break somewhere in the cable. It looks like the cable's modular, so it could be broken somewhere in there (chances are it would have been at an end). Try testing for continuity between both ends: If there's no continuity, then you have a bad cable.
B. The jack is broken inside the headphones themselves. If you can find a way to open them, then you'd be able to look. Test for continuity between the cable and the connections on the jack (hope that it's in plain sight, unlike on the higher-end Sennheisers where the whole driver unit pops out, which is a pain in itself.) If there's no continuity here, then you have a bad jack.
C. Somewhere else inside the headphones a break has formed. Check for continuity internally on the cable to the individual connections on the ears. Whichever one has no continuity is at fault.
D. The driver is dead. Like you said, using them in the garage may have caused something to happen to them. This is something that I can't exactly help you with. If you look for continuity across the driver itself, after verifying that the cables weren't broken, then something's broken inside the driver and I have no clue how to fix it. You'll need a new driver.
As far as opening them goes, given that they're a single-sided cable you'll probably run into a few issues. I can't help you much on that. If they're anything like my HD545s you have to very carefully pull U-shaped arm holding the individual ears in out of the sockets by carefully bending them outwards. Extra emphasis on the careful part, because breaking the U-piece would really suck.
Sorry for the long-winded post, but I hope this helps.
