Samsung Electronics +40%, Hyundai Motor -40%, back to break-even

I bought Hyundai Motor above 500,000 won, but it keeps dropping steadily.
I hadn't traded stocks for a while, but I entered these two stocks when the KOSPI was at 5,000; I was up about 80% at one point, but now I'm back to break-even.
I was planning to hold for 20 years, so why is Hyundai Motor acting like this?