I have two baby upper canine teeth and no adult teeth behind them. what happened to my adult canines?

Nothing.: You probably never had them to begin with. The condition is called hypodontia, which is congenital absence of 6 teeth or less. Oligodontia is congenital absence of more than 6 teeth. It often runs in families but there are other causes.

Naturally missing: Some people are naturally missing some teeth while some people will actually have additional teeth. If the baby upper canines become loose and fall out you can either get dental implants or a bridge to replace them, assuming your adjacent teeth are good support teeth.

Missing teeth: Implants, bridges are viable options. You should also consider how your teeth relate with each other. Especially if considering implants or bridges. If teeth bite together incorrectly, then this could negatively affect the implant or bridge. Consider an orthodontic consultation in conjunction with your dental options. Sometimes the spaces can just be closed with braces. It's another option as well.