You're likely within 300 blocks of your overworld portal. It's fairly rare that a nether side portal would need to be moved much more than 30 or so blocks, but it can happen. Unfortunately there is no way to calculate the coords of your overworld portal.
If you made your base near your original spawn point then a compass would lead you back. Otherwise your options are limited to either looking around (without getting even more lost) and hope you find a familiar area near your base or to put all your stuff in a chest near the portal and use F3 to get the coordinates and then kill yourself and respawn.
As to why the portal didn't connect back, it is because the portal in the nether had to be moved either due to large solid netherrack formations or large lava pools. Because each block in the nether equals 8 blocks in the overworld, if the portal was moved lets say 30 blocks that is equivalent to 240 blocks in the overworld. Portals aren't linked, when you step into one the game does a search from the relative coordinates in the other dimension for a portal within 128 blocks and if it doesn't find one it makes a new one. As you can see from my 30 block example a small move in the nether can mean a portal can easily be moved out of range of the original overworld portal.