If the teeth is not showing any symptoms and xray shows adequate root filling, you can go for a cap without re-treatment. Not all teeth need to have a cap after root canal but mostly they are badly damaged and so a cap becomes necessary. Moreover, Root canal treated teeth are weak and can easily chip off / fracture during normal eating too.
If you missed an appointment to "fill" the hole made to do root canal and a temporary "closed dressing" was only given, then possibility is that most of that "Cover" was lost and some saliva seepage occurred. This can be a problem.
Next Steps
Visit one of the dentist or your own dentist you had RCT from and take steps for cover.
Health Tips
Better a delay than to deny yourself the cap. Happy smiling