I keep getting Not Delivered in iMessage when it is delivered

This has been happening ever since my friend came over yesterday. She connected to my wifi and after she left, we texted fine in iMessage. But then a few hours later when I texted her again, it says the message wasn't delivered. But she responded to me back and says she's having the same problem. I've tried iMessage with another friend but it doesn't give me the "Not Delivered" message wih her. I've tried restarting my phone, turning off iMessage and relogging into my Apple ID but nothing is working. t's really annoying, how do I fix this?