Default [Q] Cannot get past droid with red triangle

Hi

My Nexus 7 recently ran out of battery and as per usual, died.

When i plugged it into the charger, I thought it would charge and be ready to use again.

After several hours, nothing happened. No display saying it was charging, nada.

I then went to power it up into recovery mode, by holding down power button and volume down.

This then seemed to 'reboot' the system and I now see a droid with a red triangle and the words 'No command'

Having a look around online, people have suggested holding down power button and volume up.

Whenever I do this, the screen goes blank, the Google logo appears then the same droid with a red triangle appears.

I cannot seem to get past this step.

Has anyone come across this before and if so, how I do I get past this and actually use my Nexus?

Thanks