Generally, iMessage works very well. However, after updating the iOS, some people have had issues with iMessage not working correctly.
The problems appear to be somewhat random in that iMessage completely stops working for some people while others have issues with one particular contact. If you are having issues with only one contact, it is worth noting an iMessage can only be sent when the iPhone is connected to the Internet (either by Wi-Fi or cellular data). Additionally, make sure the recipient has not disabled iMessage on their device. If the recipient disabled iMessage, you will not be able to send an iMessage to them.
Most iMessage issues can be resolved by force-quitting the Messages app and resetting iMessage on the iPhone.
Force-Quit the Messages App
To force-quit the Messages app, go to the home screen (where all the apps are displayed) and double-click the Home button to enter multitasking mode. Find the Messages app, then tap and hold on the screen image of the app and swipe upward. The image will slide up and off the top of the screen. This force-quits the app. Tap the Home button to exit multitasking mode.
To reset iMessage on the device, Go to Settings -> Messages -> Send & Receive and uncheck the email addresses in the “You Can be Reached by iMessage at” section.
Then, tap your Apple ID at the top of the screen and select Sign Out.
After signing out, make sure the slider for iMessage is set to the off position.
Next, reset the iPhone by pressing and holding both the Sleep/Wake button and the Home button for about 10 seconds. While you are holding the buttons, the screen will go blank. Continue holding the buttons until you see the Apple logo. You can release the buttons after you see the Apple logo appear on the screen.
Once the device restarts, wait 5 – 10 minutes, then turn the iMessage slider back on and sign in with your Apple ID. The iMessage activation should proceed normally. Once iMessage is activated, go back to Settings -> Messages -> Send & Receive and make sure any email addresses you want to use for iMessage are selected.
If you have multiple Apple devices, you may need to reset iMessage on all your devices. Be sure to sign out of iMessage on all your devices before signing back in to iMessage on any device.
If you still can’t send or receive iMessages with one particular contact, try having that contact reset iMessage on their device.