Hello kista1or,
You may find some helpful info and steps to try for that issue here: If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support
The "If you see a green message bubble" section will help to note the following:
If you see a green message bubble instead of a blue one, then that message was sent using MMS/SMS instead of iMessage. There are several reasons for this:
• The person that you sent the message to doesn't have an Apple device.
• iMessage is turned off on your device or on your recipient’s device. To check if iMessage is turned on for your device, go to Settings > Messages > iMessage.
• iMessage is temporarily unavailable on your device or on your recipient’s device.
If you're currently having issues sending iMessages via both cellular data and Wi-Fi, then this page can help to ensure that's activated as expected: If you can’t turn on or sign in to iMessage or FaceTime - Apple Support
Cheers.