Are you experiencing problems logging into Minecraft/Tekkit? Check to see if the Minecraft login servers are online and if it's not just you!
If the authentication servers are offline and you can't login, there isn't much you can do other than wait or play singleplayer until the issue gets fixed by Mojang. Also why not browse the forums? There are tonnes of interesting topics, related to the server. It may also be the case that they're updating to a new version.
If the login servers are online and you still can't login, post in the technical support forums and we will try to help solve your problem.
Response Code: 503 for URL: http://session.minecraft.net/game/joinserver.jsp?userMinecraft servers down