Why Does My iPhone Say I Have A Text Message When I Don’t?

Hard to tell. What kind of troubleshooting have you tried to rectify the situation?

Rebooted, I'm guessing that you mean a reset, where you hold the sleep/wake and home buttons together until you see the Apple logo and then release? If so, the next step in user troubleshooting is a restore from backup. If that doesn't work, then restore as new. After restore as new, you test the device before adding any content back.

You said you turned off iMessage and then back on. Are you sure this would have been an iMessage, or could it have been an SMS. If an SMS, it could be something at the carrier side. Contact them and ask them to reset your SMS plan.

Use this support document for restoring from backup. Back up and restore your iPhone, iPad, or iPod touch using iCloud or iTunes - Apple Support

Use this support document for restoring as new. Use iTunes to restore your iOS device to factory settings - Apple Support