This is a summary of actions one can do when Spark fails to start or login.
If you are running a HP computer and/or the HP credential manager: Update (or uninstall) the HP credential manager.
If you did update your Spark installation it may help to delete the existing configuration files (located on Windows in "%userprofile%\Spark\"). The easiest thing is to rename the Spark folder and create a new one, so one has still a backup of the chat log files which are also stored there.
Source: Rob Alexander
Check your Windows Firewall exceptions to make sure Spark is listed.