Too take to tell you now, but when you introduce potential hazards like water cooling you really must stop and check on problems when they first surface.
A few times in the past I have briefly mentioned a problem with modern computer components where there is flux reside remaining and it creates conductive shorts. These may have occurred and permanently damaged the board, or you may be able to rinse (might need gentle scrubbing with a toothbrush and detergent solution) the board off to remove conductive residue.
If you attempt this, take out the battery, remove fan(s), ideally any heatsinks particularly if the heatpipe linked type that cover so much of the board (if you have spare thermal paste or they use a thermal pad type that is reusable (silicone rather than gummy or metallic film type), and be sure to ground yourself with an anti-static wrist strap if there is even the tiniest bit of static electricity as is so often the case in winter unless your ambient humidity levels are high.
Another option is to just disconnect the PSU from AC at the wall or rear power switch, wait a few seconds, connect to AC again and see if it POSTs. This is the lower effort approach and risks further damage but the next thing to try if you don't feel like trying to clean the board off and dry it thoroughly. If the next power on POST attempt doesn't work, remove battery with AC off, clear CMOS, and try again.