Server not responding

Hi, basically I've had a server that's been running and people have joined before Today it's saying 'Server not responding' and not showing up in the server list. No-one but me can join. I checked if port 27015 was open on http://www.canyouseeme.org/, and it says

Error: I could not see your service on 86.132.199.165 on port (27015)
Reason: Connection timed out

Any idea why?