Lighting, Light Fixtures, Ceiling and Exhaust Fans - no light

Hi All,
Hoping this is something simple, but I can't figure it out. I took down a working ceiling fixture yesterday and hung a new (in the box, but turns out the wrong box without instructions) fixture from light shop with a closeout. connected the wires and I purchased a new "t type" lamp to fit it, but when I flipped the switch, there is no light. I used my little tool with 2 wires that lights up if the circuit is energized, and it light up. I tried changing the wall switch--becasue I could not think of what else could possibly explain the lack of light, and it appears that the wall switch (dimmer) is also working and energized. Please, if this sounds like something that you may have a fix for, I'd be ever so greatful. Sign me, greatful, but in the dark, silly girl. Thanks!

Just until you get it working, put in a regular switch instead of a dimmer. It's easy to ruin a dimmer, and let's take that variable out of the equation for now. Once it is all working well, you can swap back in the dimmer.

So let's clarify what wires you have. I'll guess, and you correct me or confirm. You have exactly one black wire and one white wire at the ceiling (not counting the wires from the fixture), right? You have exactly two black wires and two white wires at the wall switch box (not counting the wires from the dimmer), right?

Do you have grounding wires too?

If you suspect the fixture itself, buy a simple $2-$3 naked-bulb fixture to swap in as a test.

Thanks for your help! At the ceiling, I have 1 white, one black, and then the copper ground (which I didn't connect to anything) I hooked the fixture black to ceiling black and fixture white to ceiling white with wire nuts and hooked the fixture ground to the bolt on the metal bar that goes across the box. The wall switch box is much more complicated because it's 3 switches in a row, one to the outside porch light, and one three way switch that turns on a remote table top lamp, and then in the middle, the switch in question. There are so many wires in that box, that it's hard to figure out what all is going on, but there are 2 black wires at the wall switch and 2 black wires from the dimmer. One wire from the wall is wire nutted to 1 wire from the dimmer. The dimmer switch lights up when the ceiling light is "on". I discovered that I had to have a certain wall wire and dimmer wire together for the switch to light up. The switch doesn't seem quite as bright as before--this may be my imagination. I can take the dimmer off and also get a naked bulb fixture if that's what you think I should do next. I'll try it and let you know. Thanks for walking me through this. I really appreciate it.