First, try to change your password. Log out of anything that you're logged into (websites, game launcher), then if you were able to change the password, try to log in again. If you have an old Minecraft account that logs in with a username instead of an email, you should migrate to the newer Mojang account format.
If you have a Mojang account, try logging in with a different email address.
If you are not sure if you should use your email address or username to log in take a look at our article on Minecraft account types.
If you are not getting password reset emails within a few minutes, or you are sure that your account was compromised, locate your transaction ID or gift code and contact customer support