Nothing happens - The players are brain dead.

Either player could have pointed out to the referee that the ball was missing, and could have refused to shoot until it was replaced.
The Rules
http://www.worldsnooker.com/staticFiles/40/8d/0,,13165~167232,00.pdf
7.
Spotting Colours
Any colour pocketed or forced off the table shall be spotted before the next stroke is made, until finally potted under Section 3 Rule 3 (g) (iii).
(a)
A player shall not be held responsible for any mistake by the referee in failing to spot correctly any ball.
(b)
If a colour is spotted in error after being potted in ascending order as
per Section 3 Rule 3 (g) (iii), it shall be removed from the table without
penalty when the error is discovered and play shall continue from the
resulting position.
(c)
If a stroke is made with a ball or balls not correctly spotted, they will be considered to be correctly spotted for subsequent strokes .
Any colour
incorrectly missing from the table will be spotted:
(i)
without penalty when discovered if missing due to previous oversight;
(ii)
subject to penalty if the striker played before the referee was able to effect he spotting.
(d)
If a colour has to be spotted and its own spot is occupied , it shall be placed on the highest value spot available.
(e)
If there is more than one colour to be spotted and their own spots are occupied , the highest value ball shall take precedence in order of spotting
f)
If all spots are occupied , the colour shall be placed as near its own spot as possible, between that spot and the nearest part of the top cushion.
(g)
In the case of Pink and Black, if all spots are occupied and there is no available space between the relevant spot and the nearest part of the top cushion, the colour shall be placed as near to its own spot as possible on the centre line of the table below the spot.
(h)
In all cases, the colour when spotted must not be touching another ball.
(i)
A colour, to be properly spotted, must be placed by hand on the spot designated in these Rules