A head broke off a valve?
Intake or exhaust?

The most common reasons I see that are usually because there was something unhappy in the valvetrain, valve to piston contact, or in the case for exhaust valves, it can be a corrosion issue.

I would carefully inspect the cam & lifter that correspond to that valve, along with checking to see that the spring hasn’t given up.

Also, I’d look for witness marks in the heads near where the pushrods go.

It’s pretty rare that a valve breaks “for no reason”...... as in just fatigue.
With some detective work, you can usually find the reason.