If your phone doesn’t boot into download mode and you have a black screen after flashing froyo, here’s how you unbrick it. It has worked for many folks.
- Prepare odin with the firmware u want to flash (in my case was gingerbread 2.3.3) and get it ready to go.
- Remove battery, SIM and SD card, next connect your galaxy ace to the computer.
- Hold Volume down + Power + Home button at the same time.
- Insert the battery and wait a bit.
- At this point Odin should have detected your SG Ace ( Yellow rectangle should appear)
- Click Start button in Odin and wait about 5 min.
If all goes well it should come back to life.
You should know how to flash firmware using ODIN.
Thanks to SuperMini@xda