Minecraft – Any useful strategies for finding a Nether Fortress

I believe, basing on ReallyGoodPie's answer, I do know something that would work most of the time.

Unless you're a devout traveler and wandered for days away from the spawn point (not too smart, a creeper blows up you and your bed and you're back to spawn, days away from home - not to mention all three strongholds are between 640 and 1152 blocks from the spawn) we can assume your Nether Portal was somewhere within, say, 3200 blocks radius from 0,y,0 (that's like 9 maximum zoom-out maps!). Since coordinates in Nether are an eighth of the Overworld, that means the portal will be within 400 blocks from 0,y,0 (and you can estimate the radius by estimating the distance of the overworld portal from spawn.) You can also estimate the altitude (y) - 64 if that was the surface, more for hills, less for underground.

Now locate 0,y,0 using F3 and then travel in spiral around that point, trying to explore increasingly wider circle of terrain and controlling your distance using the coordinates. That way you shouldn't miss any terrain and be able to locate familiar areas and eventually the portal - 25 chunks radius isn't something beyond hope for exploring, and most people will have their homes (and portals) within 1000 radius, that's mere 8 chunks of Nether away from Spawn. (and if you remembered rough direction of your portal from spawn, using the Cobblestone Compass trick you can head straight for your portal from the "origin")

Another option, inspired by Loren: If you have enough wood, or a chest - or are near enough from the portal (say, aforementioned 0,y,0) - or are patient enough to locate a treasure chest at a fortress - write down your coords, dump all your equipment, suicide, then go, retrieve it all - stopping long enough to take note of the portal location this time.

Minecraft uses some basic math to determine what nether portals should link with each other. In the overworld, a portal will check if there is a portal in the nether near its coordinates divided by 8. This is to make the nether feel smaller than the overworld. I do not know why your portal suddenly isn't working, but I bet your problem would be solved if you went to your portal in the overworld, divided it's X and Z coordinates by 8, and built a portal in the nether at that location.