iPhone - Blank Screen

hi, i'm having a problem with my iPhone 3G...

whenever i press the button on top of the phone to lock it the screen goes black as normal, but i can't unlock it when i press the top button again, and when i press the Home button nothing happens the screen just stays blank.

However once the screen goes blank if i plug it into the wall charger the screen flashes on and off a few times and then it works fine...until i lock it again...

is anyone else experiencing this problem??

how do i fix it?

any help appreciated

thanks!