HODL
If you invest, it's usually for a long time (sevveral years or longer). Ups and downs are just part of the cycle, so just ride them out.
If you speculate, though, that's a totally different situation. I don't know enough to answer how a speculator should handle downturns in crypto, so I won't even try to answer.
So before you look for answers, you need to know why your spent your money on crypto (or anything else, for that matter). That answer will determine how you should proceed when the trendlines on the charts go south.