If your phone doesn’t boot into download mode and you have a black screen after flashing froyo heres how you unbrick it, it worked for me .
Step 1 – Prepare odin with the firmware u want to flash (in my case was gingerbread 2.3.3) and get it ready to go;
Step 2 – Remove battery and connect your galaxy ace to the computer;
Step 3 -- Insert battery holding Volume down + Power + Home
Step 4 – At this point Odin should have detected your SG Ace ( Yellow rectangle should appear);
Step 5 – Click Start button in Odin and wait about 5 min.
If all goes well it should come back to life.
Get the roms from sammobile site.
Be sure that you download de correct firmware for your device version.