What failed 48re?

Built tranny about 5 years of torture has a lock up switch. Got it really hot pulling a heavy load up hill. Burned the fluid badly Drained and filled no chunks in the pan but I think the discs may be worn out after all those years. The lock up switch no longer works but it is grounded properly and no issues found in wiring. Grounded wire under truck at the plug in after the resistor and nothing so what failed? I find it hard to believe the TQ is shot but what do you think. Something simple like the Lock up Soleniod shot? OR?

Thanks for the opinions!