Error 21 problem while restoring

When i updated iOS 7 last week the phone was working great and when i was using my iphone today suddenly screen got blanked and didnt restart but i tried force restart, it didnt happen..Finally i gave my iphone to my friends service center he told error 21 occurred while restoring..Need to check it deeply..I really need to know y it suddenly blacked out and y error 21 is occurring while restoring???Wat is error 21?

I need a solution clearly..