I did not invest in the stock market properly in the last decade (mid-40s; tech income but just put it to paying off house - estimating this cost be at least 5

I did not invest in the stock market properly in the last decade (mid-40s; tech income but just put it to paying off house - estimating this cost be at least 5 million bucks conservatively). Opening a stock account and will slow down paying off the house.