CSGO don't launch after upgrading operation system


I own csgo for a very long time (3 years) and it always worked fine. Lately, i upgraded my operation system from 32 bit to 64 bit (windows 7). Than, I downloaded Steam and csgo again it worked fine, but when i try to launch the game it just never works. Than i try again (to launch the game) and i get a message that says “app already running”. It’s running on task manager as - csgo 32, which is obviously not fitting for my computer, after upgrading it to 64 bit. So, what can i do to make the game work normally as before after upgrating my operation sysytem from 32 but to 64 bit? I would really like to keep playing! please answer me as fast as possible, will be much appreciated.