Kathleen, have you tried pulling on the battery? If you can remove the battery somehow and it slides on and off the charger without a problem, this will get you by until Ego is able to deal with the issue.
Make sure your tools are registered and then send them an email describing your situation:
Hi Kathleen! Thanks for your post, and we are sorry to hear that your battery will not release from your string trimmer. We'd like to assist you directly with this issue, so if you could either send an email to email@example.com with your phone number, one of our EGO specialists will give you a call back today. Or, since you already created an account if you would rather call us at your convenience and just give your first and last name, any one of our EGO specialists will be able to pull up your account and assist you as well. Please have the serial numbers off of your tool, battery, and charger readily available when contacting us. We look forward to helping you.