Electronics locking up!

Firstly - Thanks for adding me, I look forward to spending many hours here.
So - Has anyone come across an electronic unit that freezes up and powering down won't shift it?
The power supply is intact.
I specialise in Audio but have seen this occur in other non related equipment.
Any thoughts?

Your question is a little vague?

It's a fault diagnosis that I've described clearly?
Someone else must have experienced it?

Yes, freezing or "crashing" is very common, in many types of equipment.

Not restarting after a power cycle either means it's battery backed and not truly resetting, battery backed for configuration data and that has been lost, or there is a physical fault of some sort, even if just a poor / corroded connection or dust contamination.

Computer systems crash, and much (most?) electronics has a computer system inside it these days.

Freezing is also common if 'something' is missing, and it's sat waiting for a signal of some kind.