Assuming you are in the UK.
Rain attenuates a satellite signal quite dramatically. If your setup was weak to begin with for some reason this would explain your signal disappearing in rain.
Is the dish alignment good? Is the cable good quality (cheap cable can cause all sorts of problems)? If the answers are yes then it points to a faulty LNB or receiver, the LNB being the prime suspect. Cheap enough to replace at under £25.
Be sure to buy the correct one i.e does it have several outputs and if so is it feeding a distribution amplifier or do the cable(s) run direct to the receivers. Generally a distribution amplifier requires a 'Quatro' (as opposed to a 'Quad' LNB).
Dishes are also cheap and another general rule is that a larger dish gives a dramatically better signal improvement over buying an LNB with a better signal to noise ratio.
Let us know how you get on.