You can't fix the lift right away, so go back to where you left Thanos --
-- to find he's escaped. No need to panic, though; he's gone upstairs to act in the play, and he's left the judges' stones behind. Take them.
Go back to the judges and zoom in on the panel where you can see a scale. As Nancy says, one of the stones is heavier than the others, and you have two tries to find out which it is. Read to the end of this hint for explicit instructions.
To compare the weight of two stones, put one in each side of the scale and press "Judge."
If you had infinite tries, you could weigh each stone against another stone; if you had three tries, you could weigh 4 against 4, then 2 against 2, then 1 to 1 to find out. But you only have two tries (though you can start over with no repercussions).
The trick here is to weigh 3 stones against 3 stones and draw a conclusion from that about the stones you didn't weigh.
I usually put three white stones in one side and three black stones in the other side, just so I can keep track of which stones I've weighed.
If the two groups of stones are even, the heavy stone is one of the ones you didn't weigh. If one of the groups is heavier, the heavy stone is in that bunch.
Put the light stones back in the holder. Now put one of the possible heavies in one side and one in the other, and put the third one back in the holder, but apart from the others so you know which one it is.
Again, if the stones are even, the heavy one is the one you put aside; if they're not even, the heavier one is ... well, the heavy one.
Put the heavy stone in the slot at right, and the judges' box is now open.