How do you fix this? Stair step collapsed!

[Yanni]

On of the stair steps collapsed, I found out a piced of wood that held it in place had been like paper.
It was the original wood from 1906!.

How do you fix it as it is the second step and have not great deal of access.
I am thinking to compact some sand at the bottom with mortar and add bricks on top with mortar
where the planks will seat on...

 

[Wayners]

Get under.

Either by taking apart and getting under or removing whatever is under the stairs.
Lash on plenty of good quality wood glue. Screws and timber.
I've done a few and all good. Not always the best looking but I'm not a joiner.

Here is one. I got my head and arm through gap to fix tread underneath.
You can see one spilt top left on step.
Was others.

 

[Wayners]

I screwed that timber sat on the stair to underside at that point with glue.
Also timber around the edges to support step.
All easy enough