Lighting, Light Fixtures, Ceiling and Exhaust Fans - Dimmer Switch Problems

I recently replaced out an old dimmer switch in my hallway with a new dimmer. The hall light is set with two switches, one downstairs (the dimmer), one upstairs (standard switch). When i did the install,it worked fine, but when I changed the switch upstairs from "on" to "off", the light would not turn on. I went back in and tried several different combos for the wiring, and I could get the light to work when the upstars switch was on or off, but not on both settings.

In the wall downstairs, there is a red wire and two black wires. The dimmer has a green and two black. When I connect the green to red and the blacks together, the light works when the switch is in the "on" position upstairs. When I connect one black from the dimmer to red and a green to black and one black to black, then the light works when the upstairs switch is in the "off" position. Please help! I've tried everything I can think of and nothing is working.

You installed a single pole dimmer, you needed a 3 way dimmer.
Most likely the blk and red in the one cable are your travellers. The black from the other cable is the common. Try this and let us know if it works.

If I put a standard switch in there, instead of the dimmer, will that solve the problem too?

You would still need a 3 way switch. Three way switches work in pairs. You need one at each end for proper operation.

When I connect the green to red That's a suicide attempt. Don't guess.

Darn John, you just made me realize how tired I was last night when I didn't say anything about that.:o