So the rim now sets inside the tire. The someone was ME!
You have to take both sides of the tire off the same side of the rim. So, lever one side of the tire back into the center of the rim, then take it off the other side. Use some tire lubricant, or dish soap and water to make the tire bead slip over the rim easier. Good tire irons help. Never lay the brake disc directly on the ground or against anything - they bend easily. Support the rim up off the ground with a couple of pieces of wood or another tire so the disc isn't touching anything.
No big deal,,,just takes bit different effort. Put One side of the tire back onto the rim. Most will go on 1/2 way 'round or more by hand. Go ahead and lever it fully onto the rim. The Other side is Already OFF,,and will remain off,,,right? So to REMOVE the Tire,,,, Push the side you just put back On across the Rim to the opposite side of Rim. Keep one side of the tire down in the center of the Rim,,,and begin to lever the tire Off the Rim on the opposite side You started from. BOTH sides of the tire must come Off the Same side of the Rim
Try standing up the tire and rim, shove the rim down as far as you can in the tire, grab the top of the rim and try to pull it out. Lots of soapy water will help too. Gotta say, I have never seen this trick before!
