What is market is down for years/ decades?

I’m new here so pardon my ignorance. wanting to invest but when I look at the Great Depression, stagflation in the 70s. The dot com and mortgage crises. It looks like there have been several years and in some cases decades of the market being down. What happens if we retire but we are only 3-4 years into a 15-20year stretch of a down market? I know we hope for the best but I fell like this is a real concern since it has happened more than once? Any advice or opinions on this? I know we are looking at 30 years from now. But a 20 year window is a long window not to retire.