First of all go to start page https://onthe.io/auth.
Your login is almost always your corporate email, less often it’s your personal email. Try to remember which one did you use in IO.
To restore your password follow Forgot password link and put your email there. All the instructions will be sent to you within a minute. Use a link in the email to create a new password.
If you did not receive an email please check your Spam folder, sometimes they end up there.
In case you got a message that this email is not registered in IO:
Make sure this is the email you used in IO before.
Make sure there are no typos in spelling.
If you’re 100% sure in 1 and 2 - then it looks like someone deleted you from IO. To restore your account please contact your dashboard admin. If you do not know who is your admin please contact firstname.lastname@example.org.