I made a really good castle in creative mode but had not slept in a bed on superflat mode. Someone killed me and that somehow reset my spawn more than once. I did tests in a new world of the same kind of circumstances and found that when killed and not having slept in a bed you respawn in a totally different area. If anyone knows how to get back to my original spawn point I would really appreciate the help.
Use Minutor, or some other mapping program to work out the coordinates of your epic castle (in Minutor, the coordinates of the block under the cursor are shown in the status bar at the bottom). Then in-game, press F3 to see your coordinates and walk to the coordinates you found.
Then sleep in a bed and reset your spawn!!
Honestly, all I did was follow a compass. I too, was stuck and I had absolutely no idea how to find my whole city I built. All I did was follow a compass and I was back at my city. I went pretty far from my city and it took a while to get back so don't get impatient. It might take a while to get back to you house or whatever it is you built, but you will get there.