After pressing the log in button it would said updating minecraft and after that it said
Failed to launch
Fatal error occurred (4): null
I have forced an update and updated Java but it still doesn't work. Not even the demo works.
It's likely your antivirus/firewall. Apparently Avira 2012 and McAfee have been known to prevent the connection to Minecraft update servers.
If you're comfortable doing this, you could try temporarily disabling your antivirus while Minecraft updates.
I had this problem before, and I had to delete my
For Windows: Win + R. Then inside the text box type