There is an icon in my notification bar that looks like a no entry symbol (far left):
Does anyone know what it is/means? I have been unable to find out.
You've your phone on Total Silence (one of Do not Disturb modes).
In the section Turn sounds back on (turn off "Do not disturb") of the article Silence your device with "Do not disturb", Elena notes:
If your phone is in "Do not disturb" mode, you'll see an icon in the status bar at the top of your screen:
Here's further proof:
(Click image to enlarge)
Next time when you're in a similar situation, try these simple techniques I listed here.