How can I be able to message again using IMessage

Device or computer? What happens when you try? Error message(s)?

IOS

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

If you can't send or receive messages on your iPhone, iPad, or iPod touch  

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.

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.      

OS

Messages – Can’t log in.       

Messages – Can’t log in 2.    

Messages - If you can‘t sign in to your iMessage account or FaceTime on your Mac.    

Message activation error.    

Error Occurred During Activation for iMessage or FaceTime on Mac