I have a copy of the file in STL format and wondering if it is possible to convert the STL back to a TCW file?
Yes, But, you may be disappointed. do you not have a good backup of the TCW file somewhere, backups are usually in windows - documents - TC version, but the backup may just be a copy of your 'bad' file.
When opened in TC the STL will be a single object as a TCsurface object, if it was supposed to be multiple objects, you can explode once and break it into separate objects (always check selection palette to ensure it explode to objects and not polylines). The drawback with stl, is that any curves will likely be faceted. You can click the object and in properties change it to Solid, click ok when the warning message appears, but curved surfaces will remain facetted, and I doubt they will look good.