Okay, it's really easy. First, get the seed of the world you died on. Second, use that seed to make a new map. Third, check the coordinates of the spawn on that new map. Fourth, use those coordinates to locate your initial spawn on the map you died on. Hope this helps!
Next time use a bed. Superflat has only one real problem (at least the only one with no workarounds). You can't set the height of the world, so if you need to dig, you can't.
@360guster: Try to stay on-topic. This is a thread about superflat, not The Nether. But, because you are new, I will tell you the answer. The Nether is the Hell Dimension. You get sent there for all eternity if you grief 100 people.
Actually, I have no idea what the Nether is for, other than for getting blaze rods for potions and The End.
I understand what your saying.. But next time you should do things like: sleep in a bed, go into peaceful mode and write down the co-ordinates of where you build. If you do these things you can avoid your problem.
This is so annoying i like flat land but the 4 blocks of grass is kinda annyoying when im making redstone things.i had a bed and i slept in it then i died and spawned somewhere really far and i dont know where my things are also my world was my creations world so i had like a hundred awesome creations and redstone machines and traps and all that sorta stuff im so pissed.