Samsung Galaxy Ace GT-S5830 neither goes into Download Mode nor RAM Dump Mode My Samsung Galaxy Ace GT-S5830 has just “hung” during the start-up screen with the white text “Samsung www.samsung.com" on a black background.

I have tried to restart it and to take the battery out / put it in again but the result is the same. It neither goes into Download Mode (vol down+menu+power button) nor RAM Dump Mode (vol up+power button).

Does anyone have any suggestions or advice on how to fix this?
Plug it into your computer and see if by some offshoot chance you have adb available.  If so, run a adb -d reboot-bootloader to reboot into download mode.

If that's unavailable, you are SOL and will have to have Samsung repair it.  Honestly, if it's hard bricked, you're better off buying a new phone...