First thing is to check the error itself, it will be displayed on your FTP client window. This article will help you solve the most common errors:
530 Login Incorrect
If you are seeing a 530 Login Incorrect error while connecting to your FTP, make sure that you are using the correct FTP details. If you are sure that you are using the correct credentials, it is highly recommended to reset your password. A step-by-step guide for that can be found here: How to change your FTP password on hPanel?
20 seconds or Connection Timed Out Error
In most cases, this error indicates an issue with the Internet connection of your provider. If that’s the case it’s best to follow this procedure:
Reboot your router (and other network devices, if any). If the error persists, contact your Internet service provider.
Increase the connection timeout in the FTP client you are using. In FileZilla, this can be done via Edit → Settings:
You will be presented with an options menu, where you can change the timeout interval to 9999:
That’s it! Now your FTP should work without any issues.