If you can't login and don't even get the username/password dialog box, then most likely your current network is blocking ports 2083 and 2096, which are used to access cPanel and Webmail. Most office networks and hotels have a firewall setup to block unknown ports. In these case, try using the so-called proxy-subdomain to access cPanel, Webmail and WHM through the regular SSL port:
You may get a security warning, skip it and reach your destination.
If you get the username/password dialog box, but your login attempts are not accepted, then be sure that your hosting account is active (not suspended):
If your hosting account is active, then try to reset the password of your hosting account from your admin panel:
IMPORTANT: server will not accept easy-to-guess passwords and will reject it, please use long password that mix numbers with upper and lower case characters
For Webmail, make sure you are using the full email address as the username and the password of the mailbox, not the password of your cPanel.