Unable to login grafana 4.5.2 showing Unexpected error

I am using grafana 4.5.2 from last 4 month. until today it works fine but now i am not able to login in grafana showing unexpected error. Then i tried to access it using API call and they worked.
Can anyone give solution on this problem?

can you check grafana-server log? usually located at /var/log/grafana/grafana.log

Problem is solved now,there is no space left in /var so because of it this error occured