Steam dont recognize the games i already have installed what can i do ?

Well that random.
Try support?

verify?

This may work, but no guarantee. CLICKY I had this issues with my hybrid hard drive, it would no longer recognize (SSD) D drive. Even after trying this "solution" it didn't work. So the only thing I could do was go into the file & make short cut to launch the game. Or I haven't gotten around to trying this myself, but maybe click reinstall then stop the download & copy/paste the complete files into the new one, then verify the game cache..

Do cache verify once. Maybe some corrupt files cause issues and will be replaced.

I had that happen with Fallout 4 went to play it and it said it was not installed,i just used verify and then it fixed itself,but if you have deleted the files or other wise then you will have to start another download

Also using verify sometimes makes it seem like it is downloading it again so let it runs it course and after that it should put it back in the library as long as all files check out,any missing/bad and it should re-download them.

thankyou to everyone