What mode are you using? light-of-sight or radio signals?
How are you controlling that speedlite? Via a slave flash, or ST-E3-RT radio controller?
Once we have more details, we can then help to troubleshoot.
The led indicates red only. Not green. Hence not firing. Also an exclamation symbol appears on display along with ettl
I don't know why it's not firing, but I think you're misinterpreting the LED. Check your manual. IIRC, red means that the unit has reached its full firing power, while green means that it hasn't. (Yeah, I know that's counterintuitive, but it is what it is.)