Starter issues

[Hammerhead]

With the cold weather setting in, it is possible that you have a condensation or moisture issue that is freezing, preventing the bendix from engaging. The bendix is the gear/shaft/spring assembly that shoots out to engage the flywheel.
I'd start with a few hammer taps, and repeated usage of the starter in the hopes of clearing the moisture.
A little tip for those that encounter this issue unexpectedly...
This is prone to happen at times in humid environments that get cold quickly. The moisture seems to be attracted to the cold metal components creating this issue.
I had this happen to me quite a few years ago, loaded in Houston when it was very muggy. A cold front was moving down from the north and I headed north. Next morning this exact issue happened to me. Whacked the starter a few times, and never did replace that starter. Sold the truck 8 months later with it still working fine.

If that doesn't do it, I'd replace the entire starter if your doing anything.