When I try to login with my Mojang account it says "Login Failed". Why does it do that?
Did you buy a legitimate copy or are you using a cracked version? This is a known issue with cracked copies of Minecraft.
If this is not your problem then there really aren't many other reasons.
- The login servers could be down.
- You're typing something wrong, recover your password just in case.
If these aren't the problem, then email Mojang. Also, check this site for info on if the servers are up: http://xpaw.ru/mcstatus/.
This can happen quite often, and it ofter works for me to go to minecraft.net while logging in. This may only work with Tekkit, and check your internet connection works.
Remember when you use a Mojang account that you must use your e-mail and not the username you used to use for your account originally when logging in with the client.