weird switch behavior

Certainly a pickle. Have trouble understanding your terminology. Sounds like you're saying that when you turn the switch on, all the power goes off, which sounds like a short to the neutral side. Would expect this to blow a fuse or trip a breaker. You also don't say what sort of tester you're using. If you're using a digital volt meter, then with the high input impedance, your "hot" side may not really be hot to begin with. You don't say if the wiring gets hot (temperature wise). Either way, I'd be getting someone who knows what they're doing (ie. a licenced electrician), to check out your system quick smart.

How are you testing your switch? If you are using a tester with leads and testing across the switch when it is on then you will not show any reading. You need to test from line to ground or neutral and from load to ground or neutral. You may have a loose neutral connection from when you re-connected your light fixture, so while the switch may actually be energized there is no return on the neutral. I would re-check your connections.

This is not a neutral issue. If the voltage is dropping out when you connect the load, ie. "turn on the switch" and you measure no voltage on the line side of the switch, then you have a loose connection on the line side. The loose connection will be at that pigtail splice, or at a point where the hots are junctioned somewhere along the branch circuit, or may even be at the breaker (or bad breaker). This happens because you are connecting a load on a loose connection. The current connecting at the switch is amplifying your problem.

Sounds like the breaker blew

But Mark take care 'cause who knows what else might account for it. For instance you might have connected the common of your instrument to a faulty "ground"

???? leaving some of us in a sort of quandary

No offense Mark but please don't electrocute yourself