My iMessage keeps saying activation unsuccessful, but I’ve been using it fir a while before it started this

See the Apple Support article(s) below.

Messages - If you get an error when trying to activate iMessage or FaceTime.  *

If you can’t turn on or sign in to iMessage or FaceTime.  *

Messages Troubleshooting.   -

Some things you can try. Do you have access to another WiFi network where you can try? That has worked for several people.

Try a restart.

Restart your iPhone, iPad, or iPod touch    *

Try a forced restart. See the section towards the end of the article.

Reset Device. -

If necessary

Hard reset.     -

Do a backup and try a factory reset. Then restore from your backup.

Restore to Factory setting.      *

Restore - If you see an error when you update or restore your iPhone, iPad, or iPod.   *

Backup - Restore your device from an iCloud or iTunes backup.     *