Why do I can't unbricked my android?

I flash a custom rom two times and when i restarted the phone it stuck in the SAMSUNG GALAXY ACE logo and i search it in the Youtube and Google many says it is bricked. I try to unbrick it and they say go to download mode using volume and power buttons but i can't go enter to download mode even using adb. Now . What do i need to do to get back my android do i need to open up my phone? Please?