Make sure you’re entering the correct username/email and password combination. 

On desktop, a common mistake is that maybe the wrong server is selected in the sidebar. Make sure your instance is selected and active while entering your password in the login form. 

On mobile, check if you are using the correct server, by clicking the back arrow multiple times until you are presented a choice between connecting to a server or joining the community. You must click "Connect to a server", then type your server's URL.

If it still doesn’t work, you can try to reset your password (if that feature is enabled) otherwise contact your instance administrators.