Hi,

I have a big problem i hope someone may be able to help with. This is
a bit complicated, so please bear with me while i explain the layout...

I have a three-way switch that controls three lights (one light is
located downstairs by the entrance, and the other two are just up the
stairs from there...the 2nd floor hallway). The first switch is
upstairs and the second is downstairs. The down stairs switch is
located next to two other switches, each on a different circuit
breaker.

Everything was working fine...until i attempted to install some
dimmers. I had accidentally used the source from a different circuit
for the second three way switch (i connected it to the common of the
2nd switch). Need-less-to-say, when i turned the breaker back
on...BOOM! The main breaker tripped. Ok, so i got a new swtich and
this time correctly installed it (travels to the 2nd switch and common
to light fixture wire). I CONFIRMED that 120V is leaving the 2nd
switch heading towards the downstairs light fixture. The problem is,
now none of the lights work. No current seem to be flowing to the
light bulb. Any ideas what happened here?

-Since i know 120v is being sent from the 2nd switch (via the common),
How can i check continuity between the 2nd switch and the light
fixture?
-Is there a way to send a signal of any kind from the switch node to
the fixture and test if it gets there?
-Also, what is the proper way measure the voltage of light fixture.
this light fixture has a bx cable with whith and black wire. The way i
have been testing using my multi-tester is touching the red probe to
the black wire, and using the fixture itself as the ground. Is that
correct?

Thanks for any help you can offer (i am clearly not an electrician...so
please explain in layman's term).