Regular descaling is essential for the life of the shower and it’s parts. Calcification is the main cause of a shower not working. There are several descaling solutions for sale in supermarkets or DIY stores. Upon check-out the shower head should be left completely free from limescale and the water should run freely.
When the shower does not work or there is no hot water it can be for the reason that calcification is stopping the flow of the mix of water into the shower. Once this happens it can also lead to boiler failure as the heat of the water cannot be disperse through the pipes.
Loosen the shower head and look inside. Sit the shower head in a vinegar solution or some "Toilet Duck" liquid – leave it there soaking for as long as possible. Rinse thoroughly under the tap before screwing it back to the shower hose.
Sometimes the shower head is beyond repair, they are not made to last forever. In this event, take the old shower head to the DIY/Plumbing store to ensure you buy a replacement with the correct fitting.