my friend's iphone 3gs is freezing on startup showing a black screen and apple logo, it never starts, and when i plug it in the computer, itunes doesn't recognize it, how to fix it?
Go to recovery mode and reinstall the software.
how to go there? according to the instruction i should see a slide but i see nohing when trying to turn it off
Connect the iPhone to a computer that has iTunes on it, and have the latest firmware of that iPhone downloaded on that computer. Force the iPhone off, then press and hold the home and power buttons for 10 seconds. After 10 seconds, release the power button. You will see something called DFU mode. Install the firmware from there.