The most common problem when signing in Gmail is forgot gmail password. There are many reasons can make it happen, you rarely have to type your password because your web browser automatically remember your sign in information, you change your password to a stronger one and you forgot it, someone hacked your account... (see how to create a strong password)
You can recover your gmail password with the information your provided when you sign up a new account: your phone number or your current email address, it's easy and fast.
But if you cannot access your phone or current email address, it may be harder and that's why gmail account login is here :)
