i turned off my phone to save battery power &left it alone for like an hour& tried to turn it back on, the AT&T logo showed up, along with the galaxy s logo, but the galaxy s logo just stayed on the screen for awhile &then the screen goes black. the bottom keys light up& they work when i push on them, but the screen is just black &nothing happens. also, it keeps vibrating over &over again. my screen is cracked too, btw. please help me.