Hi Moira,
Thanks for reaching out to us! Unfortunately we are not able to provide support in the pre-sales forum 🙁
We are happy to assist our premium users with support issues via our premium forums, please log into the account that has been created when the purchase was made (seems to be your case as recurring events is a feature of Events Calendar PRO, a premium plugin). If you have not purchased one of our premium plugins, you can post in our open source forum. We review that forum weekly, mainly for bug reports.
Regarding the problem you describe seems you can start off by deactivating Events Calendar PRO. THen use a plugin like Bulk Delete to help you out deleting all the events of the series. Once it’s deleted, it’s probably a good idea to make a backup of the current state. After that try to re-create the event series you deleted.
Hope that helps,
Best,
Nico