In order to get back into Midgar once you’ve left it during disc 1, you need to obtain the Sector 7 Slums Key. This is a relatively simple task but there are quite a few steps.
First, you’ll need to evaluate the situation by learning there is a problem. Go into Midgar and you’ll that the door to enter is locked. Unless you’re parachuting in during disc 3, you’re going to have trouble. You will find a locked door and an old man. The old man tells you the key was lost during a recent excavation. Judging by the fact that there is only one excavation site, we’ll go there to look further.
Head over to a place called Bone Village which is a tad to the Northeast of the middle of the map. More north than east. Discuss your problem with the foreman and select the option of “I want to find normal treasure.” Now set up your excavators near the northern area of the village and detonate it.
At this point I suggest leaving to fight monsters on the world map or maybe you could even work on getting your Gold Chocobo. Whatever you end up doing, make sure you do it for a little while. This point, like many others in the game, involves a lot of waiting so just try to make the best of it.
When you arrive later to excavate your findings, you’ll have to dig quite a few times but eventually, you will dig up the Sector 7 Slums Key. This is a key item so do not worry about losing it. Now that you have this key, head back to Midgar and you will now be able to enter the door that was once locked.