As per my understanding you are getting display while installing OS and booting, however, display disappears only while getting in to Desktop.
I suspect the resolution is the reason behind the issue. Your monitor is not supporting the resolution supported by your latest board and graphics card.
I suggest to try and perform the below mentioned steps:
1. Boot the system in safe mode and check whether you are able to work or not ?
( How to log in safe mode- Press F8 while booting , select safe mode ... )
2. Check with different monitor, preferably any latest monitor which can support higher resolution.
Thank you very much for your advice. It really was the settings of the monitor. Now everything is running perfect. Thank you very much for your help!