We have lots of ideas, but there are limited options for your issue. Lets use some logic;
You have a lot of film at the end of the roll that was not exposed.
Therefore it didn't get any light on it.
This can be because a) the shutter didn't fire, or b) it was never brought in front of the film gate.
If a) it needs a service
If b) it can be because i) you didn't wind the the film on when you thought you were or ii) the gears were slipping.
If ii) it needs a service
If i) then it is likely user error, but it could have jammed somehow making you think you were at the end of the roll (my M6 sometimes does this, but I don't think the iii has the same mechanism)
So, we need some more information:
What made you think it was the end of the roll? Did it A) not wind anymore? or B) was the counter at 36?
If A) then either see a) above, or your film jammed.
If B) then it's user error or see ii) above.