2007 C3 getting no power/won't start

I was driving my 2007 C3 at around 11am, then when I've come to start the car at 4pm I'm getting no power at all to the car. When turning the key, the car doesn't even attempt to start. The radio, lights, etc do not work at all and are also getting no power.

I have tested the battery and I'm getting 12.7V from it which I believe is fine?

I have also tested all of the fuses in the engine fuse box and all are OK.

Any ideas on what the problem could be and the next step to take?

Check the basics like battery leads and all fuses, some are hard to find.
My diesel 1.4 HDI done exactly this and it turned out to be the fusebox in the engine compartment was faulty.
There is a post in here in the Diesel section.

C3 Fuse Box Locations and includes links to the fuse box ratings.

Check particularly the engine bay fuse box - F10 (30A) - refer to the diagrams in the links in the topic above.

Did you check for a dead cell on the battery?

It is possible to have 12v showing with no load, but as soon as you try to start the voltage will drop to very low.

Put the multimeter on the battery, then get someone to turn the key. If the voltage stays at 12v you have a system fault.

If the voltage just drops instantly you have a dead battery cell. It's new battery time.

I had a dead cell at the start of spring.

Battery read at 12v but a whole heap of crazy appears whenever you tried to turn on the ignition. Warning lights flashing, Odometer reporting errors and going crazy. Turn the key nothing, Central locking wouldn't work, then it would and you'd get locked in or out!

I was worried it was a major electrical fault, but I had the good sense to try bump start the car, I was parked on a hill, let it roll and bumped it in reverse (don't ask). Fired straight up. Drove to the motor factors and had a new battery in it within 60 minutes of the fault appearing. And all problems just vanished with that.

The only thing you will have to do is program the key fob again if the battery has gone dead.