The main reason may be that the public keys generated by the server has changed for some reason in the client. This may happen when you have reinstalled the SSH server. This changes the ssh public keys and shows a mismatch in the client. for that remove the file ".known_hosts" in "~/.ssh/" where ~ is the home directory of the user by which you are trying to login. Now retry SSH to the server. This may show a message that a new key has been recieved. Type yes. This brings the SSH session into existence. Another could be that you are pointing to the wrong server, maybe the address is different to the outside or there is a gateway. Another could be that there is a firewall misconfigured to block incomming ssh connections. A third possibility is that the ssh server is listening on another port than the standard port. A fourth is that the ssh server wasn't started on that machine.
Try selecting one of the TCP servers. TCP has better congestion control so if you are experiencing packet loss at some level this may resolve the issue. Try connecting to different servers. If the problem persists with multiple servers in different countries then its likely the issue is with your device, your router or your ISP. If the problem only surfaces with a single server then its likely a temporary issue relating to network congestion that should resolve itself in a short time. If you have any 3rd party security software installed e.g. anti-virus/firewall etc. you may want to consider temporarily disabling them to see if they are blocking the packets. Test your VPN connection from another network e.g. wifi hotspot, neighbor etc. If the issue persists then its likely an issue with your computer/device. If the issue resolves itself then its likely your router or ISP that is causing the issue. If you have a 2nd computer that doesn't have the exact same software configuration then install the VPN software and try connecting. If the issue persists its unlikely that its your computer/device and more likely your router or ISP. Try experiment with adjusting the maximum UDP packet sizes for OpenVPN (using the tun-mtu, fragment and mssfix directives).