Of late there has been a bug going around where an sms in Arabic has been causing peoples iphones to shut down and Apple have made some suggestions to fix the issue for the time being.
An Apple spokesman said: Apple is aware of an iMessage issue caused by a specific series of unicode characters and we will make a fix available in a software update. Until the update is available, you can use these steps to re-open the Messages app.
Apple have suggested the following..
Ask Siri to “read unread messages”
Use Siri to reply to the malicious message. After you reply, you’ll be able to open Messages again
In Messages, swipe left to delete the entire thread. Or tap and hold the malicious message, tap More, and delete the message from the thread.