Connection time out from gitlab

I have registered 2 runners for my project, they worked just fine since yesterday. but suddenly they stop to work. when I start pipelines, I see runner output which saying

ERROR: Registering runner... failed                 runner=^^^^^^^^ status=couldn't execute POST against Post dial tcp i/o timeout
PANIC: Failed to register this runner. Perhaps you are having network problems 

also when I run curl again its print Connection time out after a while. my server is in Canada.
these days I did nothing with the server and I wonder how this network problem happen to the server. I’ll appreciate anybody which help me to debug this issue or give me insight for starting to troubleshoot.