Not sure whats gone wrong

Any behaviour like the one you have been experiencing with the system to do with power is to deal with the root component.

In your case it is to check the power supply unit.
As it is clear for some reason, and since all of the other components of your system rely upon it to run stable.
You should of you can at least try the system with another power supply unit.
To rule out if it is the cause and has become faulty.

If replacing it or trying another power supply does not cure the problem.
The next set of steps is to look at the working temperatures of both your gpu on the graphics card and the cpu in the system.

What often catches people out here is in your bios under system monitoring with some motherboards you have a system shut down tempreture set for the cpu of the system.

If the working temps of the cpu reach the trigger point set in the bios it will trigger a shut down of your system while in use.
So you should check for this option and what the setting is in C or F numeric setting.
As it can often be as simple as increasing the value in the option to prevent what you are experiencing.

What it means is in respect to it.
Is to ensure that the cooling solution used with your cpu is checked to make sure it is running right.
For water cooling loops and blocks, that the pump of the unit is still rotating and giving a rpm report, and that it is working at the correct rpm rate quoted.

For air it is most likely much like the graphics card, that the heat sink or cpu cooler applied to it needs cleaning out from dust build up trapped between the underside of the fans and in the cooling fins of the cooling solution on both.

It never hurts to take a look and also verify the fans are working on a graphics card or a cpu cooler if using air.

If the system suffered from the same problem before the new card was fitted.
It points more to a problem with the CS650M PSU.

It`s the heart of your system though many people think it is not, and often the cause of problems related to what you are experiencing with your system at the moment Jack.

The very first component to check and verify it is stable.
If the power supply is not right, or doing it`s job right at delivering the correct power, then everything else, and all other components also fail to run or work in the correct manner.