I'm stuck in Hell, how do I get out?

Shinrai pretty much has it right for what likely happened.. Crashed game will not save the chunks so the portal never got generated and the Nether likely regenerated fresh, again, when you loaded the world back up again.

However.. easy way to solve this is go back to as close to where your portal led you the first time, and die.. You will go back to the over world. The nether will be saved -including- the items you just dropped from dying. YOu should be able to hop into your portal again and while it -WILL- generate the portal in a new location (9 out of 10 times atleast), it will be close to where it was the first time, and you can go pick up your items again..

Just do not die near fire or lava that may destroy your items when you drop them.

EDIT: To estimate where the portal will appear in the nether from the overworld coordinates, divide the over world coordinates by 8.. But remember that the portal is somewhat randomly placed from that location to be within 128(ish) units from it.

It's certainly POSSIBLE, but I don't know of any documented bug. Maybe it didn't save the nether map properly due to the crash, and when you reloaded, it recreated those chunks from the seed - I'd expect there to be no portal in that case, since that's not dynamically generated in the same fashion.

If you have some idea where you started this whole adventure, I'd just build another portal there.

If you can't switch to creative, (/gamemode 1) then just pause the game, and click on the Open to LAN feature. then switch cheats on and use /gamemode 1 then just go into tools (the axe icon) and click on flint and steel, then go into the brick icon, and scroll down until you find obsidian! (you know what to do from there) switch back to survival once you're out of the nether, (/gamemode 0) then save and quit minecraft, go back in and voila! You are out of the nether! (You have to exit to disable commands like /gamemode, /xp etc.)