the data collector set (" Server manager performance monitor ") .xml file was deleted somehow,
so i copied one from another server and imported it to the performance monitor, and all worked out.
All servers (DNS -RDP -AD_DS ....) went back to work, and that solved my problem. :)
one more question: when i was trying to fix the problem, i have stopped windows defender real time security,
but when attempted to get it back on it prompts me that i don't have admin permission, although I'm logged in as administrator.
what could be the issue?