Iphone 4 says Device Unknown

I have an iphone 4 and yesterday it shut down with 10% remaining. I put it on charge and it came up with a black screen and then the apple icon appeared. It alternated between the two screens for a good few hours. Strangely it came up with "Hello" in various languages. I swiped the screen and it then came up with "Device Unknown". Panicking i took it to my local Apple Store which were about as much use as a chocolate teapot. All they gave me was a telephone number for Apple Care Support Line. As you can guess that was pointless also. Got a stupid automated services which informed me cause i dont have a care package that i would have to spend yet another £25 just to get to speak to someone. Absolutely ridiculous with the amount of money Apple make yet they still hold people over a barrel to get yet more money out of them because they know thats the only way people may be able to get the phone fixed, total rip off.!!!!! Hoping that someone out there will be able to help .