It couldn't be the liquid cooling because if I remember correctly no cooling liquid would mean the pump would try to pump air (really bad for your setup), but it should not cause a system not to start up.
Check if all the wires are connected correctly (check the manual to be sure).
If they are and problem is still there then disconnect your pc from your power grid and leave only the most essential cables of your psu plugged into your motherboard (as well as the motherboards most essential cables). Disconnect your gfx card as well as your hdd's. Then reconnect your pc to the grid and just try to start it up. If it stays on again disconnect your pc from the grid plug in your gfx card and try again. Still starting up? Good, again turn of your pc and plug in the hdd. Continue this until you've plugged in everything and the pc is still running/turning on.
Through this route you should be able to locate the problem if it is a no go on the first try with only the essentials it is either the cpu psu or MB and my guess would be the MB.
That's my advice however since you have a water cooling setup this is a tedious and long road to go so try imabug suggestion first.