I could be many things. Didn't turn on the computer, disable onboard graphics (although, most modern bios should be smart enough to detect it and disable it automatically), not enough juice in the PSU for the new card, broken monitor pins (although usually this will result in discoloration), the question needs to be a little more specific because there could be many answers to it.
If you argue with an idiot he will drag you down to his level and beat you with experience.
I am not a fast writer.
I am not a slow writer.
I am a half-fast writer.