This has happened in a few games. I have mined a resource, like iron, and am using it to create units, then it suddenly becomes unavailable for no apparent reason. I still have the mine; it is still connected to my cities with a road, and it is still within my cultural boundaries. The game just won't recognize that I have access to it. What gives?

Happened to me also. Don't know the cause, but you can fix this by pillaging and remining/improving resource.

Did you trade it away by mistake? I never seen this happen.

The only reason I could see for this is if the road you have connecting the resource travels through enemy territory or the river connecting the resource to your city travels through enemy territory.

Also if there an enemy unit on or beside the resource.

Have you applied any patches to your game? You should be running warlords version 2.08.