IMessage not working in iPadOS 15.3

Hi Daily_llama, thank you for posting in Apple Support Communities.

Can I suggest your Force Restart your iPad as sometimes that can resolve some legacy issues when updating iPadOS. The following link will explain this more:

https://support.apple.com/en-au/guide/ipad/ipad9955c007/ipados

The same process can also help with your iPhone:

https://support.applehttps://support.apple.com/en-au/HT208386

I have also added the following links to recheck the settings on your iPad:

https://support.apple.com/en-au/guide/ipad/ipad70482978/ipados

And also on your iPhone:

https://support.apple.com/en-au/HT208386

Sometimes turning them off and on can also resolve the issue.

And the following link has more things to try when you cannot send or receive messages:

https://support.apple.com/en-au/HT204065

If the issue persists, please contact Apple Support as follows:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above but also again as follows Apple – Location Selector; or
• Using the "Apple Support" app, available and downloadable from App Store.