A "Bad Login" error is generally indicative of Mojang's session servers either rejecting your login, or the servers themselves not being online to serve your session. The most common causes for this are:
In most cases, this is caused when two players are trying to login to a server with the same Minecraft username. As soon as you Launch Minecraft you create a new session with the Mojang Session Servers that marks your identity as valid. If another player is already using the name you may get rejected from joining a server.
In some cases, you simply forgot that you logged in with your account (perhaps you launched a modpack and then tried switching back to your Vanilla game). Account sessions can also sometimes timeout after long periods of inactivity, so restarting your client can have an effect.
Minecraft Session Servers are Down
It's rare to occur, but the session servers can go down due to heavy load or maintenance. You can check the status of these servers via the following URL: https://help.mojang.com/