Alright...so dropping off a friend down in Auburn/Kent area and I suddenly feel the tranny disengage. A moment later it re-engages but won't shift out of first

(auto tranny btw). Get to a parking lot, check the codes...I'm getting P0700, basically saying that the Transmission Control Module is throwing a code.
So I got to limp the car back home on back roads, stuck in 1st the whole way.
Anyone have any ideas on cause/issue while I'm waiting to get a hold of the service department at Lithia?