Hello adrianna151!
Thanks for reaching out to the Apple Support Communities. If I’m understanding your message correctly, you’re having an issue where you’re no longer part of a group message that you were previously in. Is the group conversation no longer appearing in your Messages app?
So long as the conversation is appearing in Messages, you should be able to reply to it. If you remove the conversation from the Messages app, someone in the group will need to send a message for you to see the conversation again.
If you’re not receiving messages from the group, check to see if you had been removed. Keep in mind the group can only be modified if everyone in the group is using iMessage. If anyone is not using iMessage, no changes can be made to an MMS group message. These articles will show you how you can leave a group, and how someone can be removed:
Leave a group text on your iPhone, iPad, or iPod touch.
Send a group message on your iPhone, iPad, or iPod touch.
I hope this is helpful, and you have a great day.