Replacing a light fixture...Help!!!
Be careful there, red wires are not always substitute black wires, its not that simple. They are used for different things, to carry power back to a switch that will return switched power to where the source is, as a traveller in a 3 way circuit, etc. If there is a red wire AND a black, and you left the black wire hanging there are a few things here
1. Did you test the black wire for power?
2. Are you certain it is not attached to anything?
3. Is EVERYTHING else in the room working - other switches, outlets, etc?
CHances are, it was there for somthing. IF you took an old fixture out, was the wire connected then?
You really want to be careful of unused (potentially) hot wires in a box - the result could be a charred pile of lumber where your house used to be. Yours and or your family's safety is at stake, be sure about what you are doing with electrical - it is absolutely NOT a trial and error type of thing. We will be more than glad to help you figure out exactly what you've got for a circuit there.