Sounds to me like the nock just broke. That happens from time to time, so it’s good to pay attention to it. If a nock is fitting loosely on the string, inspect it. Especially if it was fitting well before.
The string likely broke because of the dry fire. Strings will usually give you plenty of notice when they are getting ready to retire. Frayed fibers at the string loops is a good sign.
No need to change a string that looks good.