You are able to try a few different things to solve this issue:
Check your internet connection. Try restarting your router to see if that fixes the issue.
Use a VPN connection, or disable it. As this changes your IP address, this can help you bypass any restrictions that may cause this error to pop up.
Check the Mojang Status twitter account for updates. If there's downtime, they've probably already tweeted about it.
Try logging in later. If there servers are in fact currently down, you will need to try logging in later.
Contact Minecraft Support. If none of these methods seem to work and servers should be functioning properly without downtime, it's recommended to contact support to help you get back into the game.
🎉 Hopefully these methods were able to solve your issue. If you have any issues or concerns, please contact us through the ticket system.