New computer annoyed me for a month but now it works by a work-around solution. The problem was that system suddenly freezed.
it looks similar to this report:
removing nouveau seems to fix the problem.
here is a sample conf for modprob
% cat /etc/modprobe.d/nvidia_nouveau.conf # to check if https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1690085 blacklist nouveau blacklist lbm-nouveau options nouveau modeset=0 alias nouveau off alias lbm-nouveau off