Either someone customized the template incorrectly (calling for a file which isn't there), or your template has some advanced functionality which you've configured incorrectly, or (and this is very possible) this is just a normal warning, which doesn't mean what it sounds like. In error logs, Warning is usually no big deal, unless there is an actual symptom (problem with actual functionality that is affecting you) to trace. Most servers don't even show warnings (or notices) in the logs.
If there is a problem you're hunting, I wonder if there might be other log messages near this one. I also would ask you post Forum Post Assistant and give us more detail.
Finally at a last look-over of your message before I hit submit, I wonder if I might be misinterpreting the question... from another read it seems perhaps the template is just gone from your hosting account? Or did not install, perhaps?