My desktop computer wont start the monitor anymore. It starts but it makes no connection to the monitor as if it was not connected. I tried it on several screens and figured out that it is a problem with the tower. Maybe the video card is not working anymore because i installed a Nvidia Geforce 8600 . Is there any way to fix this?

You are implying that you had another video card and that was working (?) and then you installed the GEForce 8600 and it is no longer working. If that is not correct, then you need to provide a more thorough explanation if you expect to get any help. If it is correct, then the new video card may not be working properly or maybe it wasn't installed properly.