Just had an MCB trip.

It's a 6A MCB with only 1 fluorescent fitting on it. Turned off the switch
to the light and the MCB would not re-set. I then removed the started for
the light and the MCB still won't re-set.

Anyone got an idea what the problem might be?

As the switch is off to the light, that rules that fitting out (Unless it
has be wired to switch the neutral, rather then the live!)

If this is the only thing on the circuit, then there must be a cable fault,
or the MCB is faulty.

Does it trip immediately, or is there a delay?

Have you knocked any nails into the wall recently!?

Any other DIY work done recently?

Sparks...