There was a widely-publicised internet bug discovered recently called “Heartbleed“, and many websites and services have been affected by it.
After the discovery of the bug, Mojang have recommended that all players should change the password for their Minecraft or Mojang accounts. (See this page for details.)
You can do this quickly and simply, depending on what type of account you use:
- If you have a Minecraft account, visit this page.
- If you have a Mojang account, visit this page.
Thanks for your patience and attention to this matter.