Hi there! @Jujubeehime thanks for posting your concerns.
I’m sorry to hear your SIM was thrown away. Any AT&T company store will be able to activate a new SIM on your line so you can maintain your phone number.
To locate your nearest store please go to www.att.com and select Store Locations & Appointments on the upper right of the site. From there you can enter your zip code and find your nearest store address, hours of operation and phone number.
Hope this helps!
Charise