Ram rarely fails, but that may be the case here.
Download and
Run memtest86 or memtest86+
They boot from a usb stick and do not use windows.
You can download them here:
MemTest86 is the original self booting memory testing software for x86 and ARM computers. Supporting both BIOS and UEFI, with options to boot from USB.
www.memtest86.com
Memtest86+ is an advanced, free, open-source, stand-alone memory tester for 32- and 64-bit computers (UEFI & BIOS supported)
www.memtest.org
If you can run a full pass with NO errors, your ram should be ok.
Start by testing each stick individually in a slot that will run.
If a stick shows ANY error, it is defective, and you will need a replacement kit.
Test one stick in the second slot that used to run ok.
If that fails, your slot may be defective.
One common cause of defective slots is a bent pin in the motherboard socket.
But, I strongly advise you not to remove the processor to check.
It is very easy to damage a pin when placing the cpu.
Since you used to run ok, that is not likely the issue.
Quality ram will have a lifetime warranty, but it will not be worth the hassle.
Are both sticks a part of the same single ram kit?
If not, that can be a problem.
Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, can be very sensitive to this.
This is more difficult when more sticks are involved.
Ram must be matched for proper operation.