The glass interface which makes the iPhone so popular and easy-to-use is also what makes it fragile and susceptible to damage when dropped.
It's most common for the top glass screen to smash but still be usable, although visibility is affected, especially if the cracks spread across the screen. In some cases the touchscreen beneath the top layer could smash meaning the phone's functions don't work - making it unusable.
Either way if you want to repair your phone you will most likely have to pay to get the cracked screen repaired by Apple or by a specialist phone repairing company as Apple's iPhone warranty states clearly that it does not apply to:
"damage caused by accident, abuse, misuse, flood, fire, earthquake or other external causes"
You can contact Apple's repair centre at: www.apple.com/uk/support/iphone/service or call their technical support service on 0844 209 0611 (Calls are charged at 5p per minute from a UK landline and may vary from a mobile) to see how much they would charge for a screen replace.
You can find specialist phone repairers on the internet or in your local directory who may give you a cheaper quote than Apple - estimates range from £50 upwards.
Be aware if an external company repairs your phone and it's under warranty it might make the warranty void if you have any future problems.
Internet forums discuss the pros and cons of repairing a cracked screen yourself and there are DIY videos on YouTube to help you do that - but it's not for the feint-hearted and not something we would endorse.
Unfortunately if you've cracked your iPhone screen it's most likely to make a dent in your wallet if you want it repaired. Remember to always use a protective cover which may help prevent any cracks appearing in the future.