When you leave a community, your account in that community is deactivated rather than deleted. A deactivated email can't join that community again through a Sign Up Link or invitation, so an admin of the community needs to reactivate you.
What you'll see
When you enter your email on the community's sign up page, you'll see "This email has been deactivated in [community name]". Signing in shows "Your account has been deactivated".
How to get back in
Contact an admin of that community and ask them to reactivate your account. The message shows the community's support email. Each community manages its own members, so Heartbeat support can't reactivate you on their behalf.
Once you've been reactivated, sign in with the same email. Leaving cancelled any subscription you had, and reactivating doesn't restart it, so you may need to subscribe again to get into paid areas.
Frequently Asked Questions
I deleted my information when I left. Can I rejoin? Yes. If you ticked "Permanently delete my personal information and generated content" when you left, your account was deleted, so you can sign up again as a new member with the same email.
I'm an admin and a former member can't rejoin. What do I do? Go to Settings > People > Members, open the Deactivated tab, select the 3-dot menu next to them and choose Reactivate User. How to deactivate or delete a user explains how reactivating affects their subscription.