What would happen if you broken your collar bone and never told anybody for a very long time?

No one would know. Odds are in your favor. Even without treatment, most collar bone fractures heal uneventfully. 5% of the time a nonunion or malunion would result in continued problems. If you're not having problems, then no worries. If pain or shoulder problems, then make an appt. With an orthopedic surgeon.
Clavicle fracture. The clavicle fractures that we see in practice are almost always treated closed. If I fix one a year that is alot. As dr. Degruccio stated they undoubtedly almost always heal uneventfully in 6-8 weeks. If you still had tenderness over the fracture site at 8 weeks I would be concerned. I am not really sure what you mean by "never told anybody for a very long time". Is there something else?
It would heal. Collar bone fractrues almost always heal on their own but they hurt like heck so if you think your collar bone is broken seek help to get some pain meds.