Upon leaving Midgar and on Disc 2, you won't be able to re-enter Midgard at all. However, there is a method to returning to Midgar, just return to Bone Village on Disc 2, and choose to dig for treasure. Examine the bullet shaped tip of the plane broken off. Walk below it and position yourself as close as you can between the tip and the plane itself. If you choose dig there you'll find the Key to Sector 5.

The Key to Sector 5 opens the gate from the outside world to Midgar.