Coming out of the Nether caused the Overworld to be deleted and replaced with a different world

I built myself a nether portal, entered it, but when i went back throught the very same portal i was in a cave under the ocean. I followed my compass but it instead led me to a spot in the ocean, not my previous spawn point. Using the F3 screen i returned to the coordinates of my base, which also was pointless, as its not there. It isn't even the same overworld, as my base was NOT in a jungle before. Help?