Many solutions. Which one varies with the PC that has the issue. https://www.google.com/search?q=WHEA_UNCORRECTABLE_ERROR&ie=utf-8&oe=utf-8 shows that it happens with overclocked PCs, overheating and other causes. Stress tests rarely tell you which one it is but folk like to run those tests. Here I look over the PC for it's make, model, age, then if I find overclocking, set it to stock then clean the PC and check fans too.

If the machine is defective in some way what happens next depends on the warranty. If in warranty you have them call the maker, if out of warranty you make an estimate for repair.