Hi @bobertron12720 ,
Was there a power outage or storm between the time that the TV was on before you went to work and when you got home?
If so there may have been a power surge that affected the components on the board(s).
Perhaps, if there was a storm etc, the TV may be covered by your home contents insurance policy. Worth a check.
If there was no storm or outage then how old is the TV. You say it is a 2018 model. Is it still covered by the manufacturer’s warranty period?
If so follow the directions in the manufacturer’s warranty statement (it is possibly in the TV’s user manual) about how to make a claim for a warranty repair or replacement.
If it is no longer covered by the warranty then as you have already found one problem with the TV (and there may be more, the one you found may be the only “visible” problem), without a schematic diagram (and Toshiba does not release them publicly) it will be difficult to find.
Since it is not showing any signs of power perhaps replacing the Power board may be the best option to try first and hope that it took the brunt of whatever happened and that it did not affect the rest of the TV. Was it left on when you left for work?
Here’s a link to a supplier of a power board for your model.
It is only shown to give you an idea of the cost of the board. There may be other suppliers that suit you better. Just search for 43LF621U19 Power board to get results.
Important: Check that the board number printed on the power board in your TV matches exactly what is shown in the link or from wherever you decide to buy the board.
Also if it was a storm or outage that knocked out the TV then perhaps connecting the repaired TV or a new TV (if you get one) to the power via a surge protector may be a prudent move in the future. Just a thought.