Have you tried unplugging and reconnecting all your cables? What your describing does indeed sound like a PSU issue if nothing got knocked loose somehow. I'd also check while you're working inside your PC and inspect for possible leaks for your watercooling kit to narrow that down. It's not unusual to have a PSU issue where the motherboard lights, standby LEDs, ect. still glow but the system doesn't power up normally. It's usually extreme temps or PSU issues that would cause a random shutdown with no warning. However, if it was temps, your system should start normally and at least run for a short time before powering off again.