There is no way to know when the bottom has been reached, so the best strategy here for long term investments, is to keep buying, but not all at once. Use 10 to 20% of your cash and buy some stocks. If the market goes down more the next day or two, buy 10 to 20% more. Keep doing that until 80% invested, leaving a little cash in case of a big further collapse.
It is crucial to stick to buying quality stocks, with big earnings potential, that have been discounted. Stay away from the market open.