Using a router? Connect your computer directly to your modem to see if you can sign in. If you still cannot sign in, try a direct connection with a different computer to see if it's an issue with a specific computer.
If you can log into HootSuite through a direct connection to your modem, it is probably an issue with your router's firewall. Adjust your firewall to allow logins for SSL (or the https protocol). Consult your router's help documentation on how to do this.
Sign in on a different computer from a different IP address. If multiple computers cannot sign in from the same direct connection to a modem, it may be an issue with your internet service provider.