Don't panic just yet. Its not dead (well, hopefully). I'm pretty sure that just means it can't boot into the OS, and you will need to reflash it with a computer using the appropriate utility.

Although to make sure, when you turn the phone on, make sure to hold both the volume up and down while holding power. This should bring up a menu of sorts aka fastboot mode. Does this come up at all?

 

OP we need more info.

Are you on a GSM/VZW/Sprint nexus?
Were you originally rooted?
What were you last doing before this started?

 

This, we need more information. Help us help you.

 

Is there a dead android under the triangle with the ! And is the triangle red?

 

What happens when you pull the battery, then replace battery, then press and hold volume up + volume down + power button all at the same time?

 

also more than likely its not dead. Worse case scenario you reflash the stock image in ADB. Takes 15 minutes.

 

Do you have a nandroid backup? Can you access recovery mode (TWRP or CWM)? This is a difficult phone to completely kill. Worst case scenario head to XDA, grab a stock image and ODIN, and flash the stock image.

 

You are in Download mode. dont panic, dont worry.
there are ways to use ODIN for the GSM Nexus

however i want to try something else with you first!

remove the battery, and put it back in, hold vol down and power at the same time. does it boot to the bootloader? (do you know what the bootloader is?)

give me that information and i can assist you furter!

 

Try holding volume down + volume up and power.

 

You're doing it from the phone powered off, right?

 

it is kind of odd that you cannot boot into bootloader mode, im wondering how you got into downloader mode for one

 

Broken volume rocker?

 

im not sure about that, i cant say. I am curious of as to how OP got into downloader mode.... that part is fishy

 

The exact same thing happened to me when I tried to change fonts. I pulled the battery, held the power button down for 30 seconds, put the battery back in, then held the power and volume down buttons at the same time, booted into recovery and flashed my last backup. Problem solved.