Why does the screen not light up when integrated water cooling is installed?

The graphics card may still be spinning, but it's not due to water cooling leakage. It's like this, the entire host is useless
Check the fault light on the motherboard to see where the power on self-test stopped, and then go to the corresponding hardware to see if there is any problem

First of all, make sure that your water cooling is not a problem! It is very important to have no leakage on the appearance.
Then take a look at the beginning. If the fan doesn't turn, then confirm if it's a problem with the motherboard. If the CPU FAN is not connected, the motherboard will report an error and the screen won't light up.
Then carefully check if the wiring of the motherboard's wiring is normal (such as the 24 pin large wire on the motherboard, sometimes it is not properly connected, which directly affects the power supply)
Finally, let's take a look at the power supply of the entire machine to see if there is a problem. If the power supply only works for a while and then drops, it's best to make sure that there is no problem with the power supply. If there are no issues with the above, then I think the CPU may be broken. Personally, I think the biggest problem is that your CPU fan is not connected properly, or the memory module was loosened directly when installing water cooling. I suggest you focus on these two projects, as it is highly likely that this is the problem.

It doesn't matter what kind of water cooling you installed, it's the process of installing water cooling that went wrong.
First of all, you need to determine where the problem is. It's already installed with water cooling, so the motherboard shouldn't be too poor. Generally, motherboards have fault indicator lights. If it's a MSI motherboard, it comes with it, while other brands and mid to high end motherboards come with it. This can roughly determine the problem with that accessory.
If not, we can only eliminate it step by step. If not, go to the computer store and find someone to take a look