If you want to login and you get an error "Account doesn't exist" this can be because of two reasons:
Double check if you are entering correct access key and keep in mind:
You can request forgotten access key via this form.
System regularly deletes inactive free/trial accounts to keep up with our internal GDPR rules as we don't want to hold the data if you aren't using your account. If your account got deleted because of inactivity, you can simply register a new account here.
Keep in mind that whatever you mined with your old account isn't lost as you aren't mining to minerstat account but to the pool and all coins are still on the pool (depending on the pool's policy about inactive accounts). You can continue to mine to the pool with your wallet address at any time - with our without minerstat account.