Mojang has a support article on their website that explains possible reasons you might not be able to log in:
Do I log in with my email address or my username?
There are two
account types - Minecraft accounts and Mojang accounts.
Any Minecraft account created after Fall 2012, or one that has been
migrated from the old account format is located on a Mojang account,
and logs in with its email address. An older Minecraft account that
has not yet been migrated uses the username to login.
You can learn more about the different account types here
I forgot my password
To reset the password for a Minecraft account
(you log in with your username) visit account.mojang.com/migrate.
To reset the password for a Mojang account (you log in with your email
address) visit account.mojang.com/password.
I forgot the email address used for my account
If you have a Mojang
account and have forgotten the email that you log in with, try
resetting your password. This will send an email to the address that
you have registered, letting you know which address you used.
Alternatively, try other email addresses that you own. If you cannot
get Mojang system emails, visit this article
I need to change the email address used for my account
If you have a
Minecraft account you will need to migrate it to change the email
address, to learn more please go here.
To change the email on a Mojang account, visit
account.mojang.com/me/settings. You can either have instructions sent
to your current address or, if you don't have access to that email,
you can have it changed by passing a security challenge.
I remember my email address, but not my username.
If you have a
Mojang account, your Minecraft username will be shown in-game and on
your Mojang account page.
I've tried the above, but I still need help.
Please contact customer
support through the contact form.
For clickable links, visit the article support page linked above.