You are going to have to be a bit more specific.
So, you have plugged in the display, attached any required power leads. Are you still attached to HDMI? When you turn on does anything appear on the HDMI or LCD at all? The rainbow screen?
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
Contrary to popular belief, humorous signatures are allowed. Here's an example...
"My grief counseller just died, luckily, he was so good, I didn't care."