Thermal runaway (B) is the most probable cause. The 24-hour delay suggests a gradual heat buildup. A failing TIM would cause the GPU to overheat until it triggers a thermal shutdown, resulting in the power drop and reboot. While a PSU issue (C) is possible, redundant PSUs should prevent a complete failure unless one PSU is completely dead and the second PSU is overloaded by the entire load for a short period. The other options are less likely to cause this specific failure pattern.