If you are using a router, connect your computer directly to your modem to see if you can log in. If you still cannot log 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). For directions on how to do this, call your router's manufacturer.
If nothing else works, try logging in on a different computer from a different IP address. If multiple computers cannot log in from the same direct connection to a modem, it may be an issue with your ISP.