There’s a bug going around that causes iPhones to reboot when a text containing specific English and Arabic characters is sent to the user. The text is actually a string of Unicode that crashes your phone. If you’re reading messages, your text app will shut down and sometimes won’t re-open. If the phone is locked, the whole thing will reboot.
A lot of people are sending the text as a joke, which by itself isn’t so bad, until twenty of your friends decide to do it and your phone reboots 20 times. This bug seems to only be affecting phones running OS 8.3 and later.
There’s really no way to prevent your friends (or anyone else) from sending the message and Apple is working on a fix. If your message app has already crashed, Apple suggests having your Siri digital assistant read your unread messages and then use Siri to reply to the message that crashed your phone. . You should then be able to open messages again and delete that message.