There are 3 cables going into the same box - all three have a black, white and ground. I tried connecting a dimmer switch with one black, two reds, and a ground. I identified the hot wire and connected it to the black on the dimmer. The other two red wires from the dimmer were attached to the other two black wires. The light does not work. What did I do wrong and how can I fix it?

The dimmer you have, with one black wire and two red wires, is a 3 way switch/dimmer.
I do not think that you had a 3 way switch in the box, with 3 two wire cables.

Since you know which is the hot feed, that fed the switch, were the other two wires connected? If so, then, you can use the new dimmer, just disconnect one of the red wires from the two blacks, and cap it with a wirenut, leaving one red wire spliced to the two blacks.

These two blacks must feed two switched lights.

If not, one of these two blacks may need to connect to the hot feed, leaving only one black to a switched light.

Hope this makes sense. Really need to determine the purpose of the two blacks you have spliced together.

I would cap off 1 red like above.
Test without dimmer first, then connect dimmer, misswiring can destroy dimmer.

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Originally Posted by Stratmando

I would cap off 1 red like above.
Test without dimmer first, then connect dimmer, misswiring can destroy dimmer.

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Thank you both for your help. I didn't ruin the dimmer, and I did discover that I needed to connect the other two blacks to the red and leave one capped. Thanks again.