All Limited inactive accounts get deleted after 2 months of inactivity on web dashboard. The action is made by the system and is irreversible. User might get notified about account deletion 7 days before this happens through the e-mail address they use on minerstat dashboard.
If you get an e-mail saying your account will get deleted, you can prevent that by logging in to your minerstat account on the web dashboard. However, keep in mind that the system will notify you only once. If you want to keep your Limited account, you should use it (at least login to it on the web dashboard) at least once every 2 months.
Once your account gets deleted, you can register a new account with the same e-mail address, but your new access key will be different than the old one.
Paying accounts do not get deleted by the system if they are inactive.
Minerstat is remote mining monitoring and management software - so even if you are mining and your mining client is running, you weren't using your minerstat dashboard, which represents your minerstat account. Like mentioned above, login to your minerstat dashboard at least once every 2 months and make sure everything is connected properly and your worker still displayed as online on the dashboard to prevent going into an "inactive account" mode.