Imessages
It's not working
iPad Air, iOS 12
Messages - If you get an error when trying to activate iMessage or FaceTime. https://support.apple.com/en-us/HT201422
Messages Troubleshooting. https://support.apple.com/en-us/HT204065
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 https://support.apple.com/en-us/HT201559
Try a forced restart.
Reset Device. https://support.apple.com/en-us/HT201412
If necessary
Hard reset. https://www.wikihow.com/Hard-Reset-an-iPhone
Do a backup and try a factory reset. Then restore from your backup.
Restore to Factory setting. https://support.apple.com/en-us/HT201252
Backup - Restore your device from an iCloud or iTunes backup. https://support.apple.com/en-us/HT204184
If you still have problems, have someone else attempt to sign in using their Apple ID. If they can successfully sign in and use Messages/FaceTime, it may be your Apple ID. Someone I know had this happen and I was able to sign in using my Apple ID. After a call to Apple Support and then being elevated to a tier 2 support person, it turned out it was a problem with their Apple ID. It took the software engineers 3-4 days to fix the problem. When calling Apple, make sure you tell them it is an Apple ID problem so you don't get charged.
You can also try signing in on another device/computer. There have been a couple of posts where people's device/computer was blocked. You will need to contact Apple for that problem also.
There is a contact link.
Apple ID - Official Apple Support. https://support.apple.com/apple-id
Contact Apple for help with Apple ID account security - Apple Support. https://support.apple.com/en-us/HT204169
Get Support https://getsupport.apple.com/. Work through the screens to get a chat option.
https://twitter.com/AppleSupport
Or fill out and submit iTunes Store - Contact Us - Official Apple Support https://support.apple.com/en-euro/itunes-contact The form is for iTunes, but it will get passed to the ID Support section.