My brother's PC has an i9-9900K, RTX 3070 Ti and MSI MAG Z390 Tomahawk. It won't boot, and the debug light stays on DRAM, sometimes briefly switching to CPU. I've tried one RAM stick and clearing CMOS without success.
After removing the cooler, I found no thermal paste. The PC had previously worked briefly before shutting off. Is the CPU likely damaged, or should I try another motherboard?
Start by putting the cooler back on properly with thermal paste. Make sure the cooler is firmly mounted and its fan is connected. Don't run the CPU without the cooler or touch it to see whether it heats up. It can get dangerously hot very quickly.
Missing paste can cause overheating and shutdowns, but it doesn't prove the CPU is dead. A correctly mounted cooler gives you a safer starting point for testing.
The DRAM light is a clue, not a verdict that the RAM or motherboard is bad. Power off and unplug the PC, then firmly reseat a single stick in the slot recommended by the motherboard manual. Try each stick on its own. A stick that looks installed can still be slightly out of place.
Since clearing CMOS didn't help, check the manual for the proper procedure and try again. Give the PC some time after powering it back on rather than immediately cycling the power.
I wouldn't buy a motherboard yet. If reseating the RAM and restoring the cooler don't get it to post, check the CPU socket for bent pins and inspect the CPU and socket for debris or damage. The memory connection depends on the CPU and socket too, so a DRAM light doesn't rule those out.
If you can borrow compatible parts, testing with known-working RAM or a compatible board would be more useful than guessing which component failed. Check the motherboard and CPU power connectors are fully seated as well.
Thermal protection can shut a system down when it gets too hot, but that alone can't tell you whether any part was damaged. A CPU getting warm also doesn't establish that it's working properly. Get the cooling sorted, then troubleshoot the boot failure separately.
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Be careful inspecting the socket. The pins are delicate, and forcing the CPU or cooler back into place can make things worse.