Phone won't bootup

Ok just tried that - I got the Android laid on his back with his front open and a Red exclamation mark above. Below were the Words “No command”.

When I tried to press up or down volume nothing happened.

I tried it again - same. However it suddenly restarted and booted up. The screen went to sleep so I tried to wake it up, but it crashed rebooting again.

What should I try next please?

Thanks