Plate of food stuck and invisible now in a room!

It's a common glitch in the game. @RedShoe7 got it right. If Olivia was routed to go to that room before you locked the doors, she would get through. Definitely check all the item slots on furniture. however, I highly reccomend just bulldozing the lot and starting over if you still can't find the dish. Other simmers have probably uploaded their OG house on the gallery if you need it.