Gentlmen , Help

This week end I had a interesting incident that happened .
My Sienna locked up all power , I mean totally shut down on it is own. Then slowly the power came back on when I tried to start it you can see the dash lights started to get brighter.
Is there a shut switch on the van . what is its purpose ?

just to report on my post

I was able to find new Starter relay and replaced the old one.
Initially ity did not start , then I tested the Battery , it had plenty of CCA at 514 CCA
so I Pluged the bettery again , turned the ignition on
I heard all this noise of sounding like a winding noise. Not sure but it came from the driver side under the battery .
It may be the starter
Well the car started and it has been starting every since.

Need too understand what happened

sounds like a grounding problem:

issue is if it did it once it will do it again,

At least I believe I would take the battery cables off, clean and inspect the connections.

On my chevy truck the cable looks good, but when I disconnected the cables they had corroded up into the wire, cleaned and protectant was sparyed on wires, no more problems.