The life of the super hero is a tough one, and probably a lot harder than the non-super hero. But from time to time an event or injury breaks their will to carry on the fight against a seemingly endless roster of aggressors.When this happens the Hero will usually find a protégé to train up or a new hero will move in to fill the gap or possibly the protégé will take on their masters identity completely as a direct replacement.
Some comics deliberately use the retirement of an aging hero or team of heroes as a thematic starting point. This is most notable in the series The Watchmen that uses the retired and replacement heroes as the basis for the characters and a plot that intertwines around that idea.
Retirement is a particularly dangerous act for a hero and alongside revealing their secret identity is probably the time when they re at their most vulnerable; both thought lack of readiness and age.
Characters like Superman and Spider-man have become disillusioned with their charges and left, leaving them to fend for themselves, in a kind of early retirement.