Several options are possible:

1) Entered user name or password are incorrect:

Please check whether the entered e-mail address is correct or recover your password. (If you don’t find the reset e-mail, please check your spam box).

2) You usually log in via Google:
In this case, you don't have user name or password on the site, because you log in with your Gmail account, please try to log in by clicking on the Google red button.


3) Your account has been deleted:
According to rule 11 of the website rules, if you don’t log into your account for more than a year, it will be deleted automatically.

See https://5sim.net/rules


4) IP address:

Please try to change your IP address and log in to your account again, because this problem might be connected with your IP.