The breaker (pictured above) may be an auto reset or it may have a manual reset button. Sometimes, the auto reset type will reset itself when you disconnect and re-connect the battery. The manual reset needs to have the button pushed. If the breaker won't reset, replace it.
You can test them by checking (with a test light or meter) for voltage from the posts to ground, if you have voltage on one post (the one nearest the battery) and no voltage on the other, the breaker is tripped. They are available at most auto parts stores.
Note !!!
Those breakers come in various amp ratings (as do fuses). If you need a replacement, do it with one rated for same amperage as the old one.