The reasons of signing your Gmail account page could be these;
1- You may forget your password
2- Hacking issues; Someone could stole your account already.
3- You may forgot your username.
4- Account Disable Issues; When you’re sure about your username and password is right, Google could been disabled your account because of privacy etc.
5- Change your phone number; You can’t reset your password when you change or forget your ex mobile number.
6 – 2 step verification issues
7- Browser functionality ; Your browser settings could ve off etc.