my messages have randomly turned green and won’t turn back blue. what do i do?

It's probably that the person you're messaging either has an Android device or does not have internet connection on their phone. If this is the case then your phone will automatically send your messages in green which goes through cellular rather than wifi.

If that's not the case then see if your iMessage option is switched on settings>Messages>iMessage.

Is this when you send messages to everyone or just a particular person? Have you checked in Settings > Messages to make sure iMessage is turned on?