The effective power vulnerability caused my phone to respring after showing the lockscreen page

See this article:

http://www.macrumors.com/2015/05/26/ios-bug-crashing-iphones-with-text-message/

Have you tried resetting your phone by pressing and holding the Home and power buttons until the apple appears? I haven't heard that that will necessarily work but it won't hurt anything. If it doesn't, you may have to restore your phone from a back up using iTunes.

It should not be necessary to restore a backup. The macrumors article has much simpler solutions, such as sending another text to the phone to push the offending text off the screen. Also, until this is fixed, it's a good idea to disable banner notifications for messages. This eliminates the vulnerability.

However, if the phone keeps power cycling and the user can't even get to the messages app, those options may not work. I believe Thomas suggested that, in such a case, restoring might be required. Better suggestions would certainly be welcome.

Well, I certainly trust Thomas on security. I haven't gotten a message yet, so I can't try any of the fixes. However, I've seen several posts from people who fixed it by sending themselves several texts. The vulnerability is in the banner display of texts, so once the offending text scrolls off the problem should go away. I suspect the people who have problems have been sent the message several times, and until that queue is cleared the problem can't be resolved.

in the short time that the phone turns on. Slide up and turn on airplane mode. It might take a few times especially if it keeps restarting but once it doesn't, go into whoever sent you them and once you're in the conversation with them turn airplane mode off. Then let the texts come through. When they stop you can continue using your phone. Or if you think they'll do it again click on the details and turn on "do not disturb" this will turn off notificatoons for this user and they can't cause your phone to crash again