There is no known way, within your current game - it is corrupt and should be scrapped. The only choices are replay from a previous saved game or cut your current characters out of this saved game and paste them into a previous saved game. Either way, your current position is lost - the second way your experience and levels are not lost.You could "teleport" (edit their location) your party to the other side of the wall, but this is really no good, as every time you get back to this point in the game, you would find the rock wall again. Better to "fix" it now.
Tools for your Wizardry(r) toolkit