- Short selling if you believe that it will go down even lower.
- Placing a long position in exchanges like Binance etc. This is you expect that the altcoin will recover.
- Buying it and then wait for it to recover.

I'm assuming that what you are saying is "all altcoins" collapsed including those large market cap coins. There are many ways to profit from these altcoins depending on which altcoin you will choose to buy. I'm always a fan of buying on the large market cap coins as they are way less risky than those with low market cap altcoins, but gives lower reward. On the other hand, maybe you should consider researching and buying those young and new projects as long as you are ready with the worst case scenario. Young and new altcoins can give you huge profits.