Q: why has my iMessage on my mac suddenly gone inactive?

Hello, Jimboatthebeach!

Thanks for reaching out to the Apple Support Communities! I love being able to use my iMessages as well on my devices as well, so I know how important it is to be sure it's working. Without knowing what steps you've done to troubleshoot iMessages on the device or the exact error message you're getting, you might want to try the steps listed in this article:

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

Specifically, 

Check your device settings

1. Make sure that you’re connected to a cellular data or Wi-Fi network. If you're using an iPhone, you'll need SMS messaging to activate your phone number with iMessage and FaceTime. Depending on your carrier, you might be charged for this SMS.
2. Go to Settings > General > Date & Time and make sure that you've set up your time zone correctly.

 

Turn off and restart iMessage and FaceTime

1. Go to Settings > Messages and turn off iMessage.
2. Go to Settings > FaceTime and turn off FaceTime.
3. Restart your device.
4. Turn iMessage and FaceTime back on.

Here are some tips about Two-Step verification as well:

 Frequently asked questions about two-step verification for Apple ID

Hope these steps help! Cheers!