I have no idea if your issue is/was the same issue I had after I reinstalled Conan Exiles.
After I reinstalled Conan Exiles I could no longer start Conan Exiles like I used to be able to. I would go through the Steam frontend, but when I tried starting the game, nothing happened.
I ended up resolving it by copying the shortcut to the EXE to my desktop. Since then, every time I want to play, I open Steam and then clcik on the link. Works perfectly.
I suspect that there might be some issue with Windows execution perhaps and some security/trust setting. Dunno and don’t care. The above has worked perfectly since.
Maybe others will have different solutions, but this one works for me. Good luck!
ps: If you find a solution, please add it to this ticket as others will benefit.