@Mouse For me, one of the primary functions of powers should be how they reflect the heart of the character. Don Quixote, d'Artagnan, Superman, Katniss, Tarzan, etc. Another function should be to juxtapose their amount of inner strength compared to their outer power. Samson, Achilles, Lancelot, Morgoth, Wolverine, Prospero, etc.
To my knowledge, these are the characters who had powers (or at least political clout), lost them and then regained them...
Samson.
The Bible, The Book of Judges, Chapters 13-16. Fulfilling his parent's vow, Samson enjoys God's blessing even though he blatantly disobeys all religious and health commandments. He loses God's favor when he divulges his secret oath regarding his hair. Shorn, blinded, and enslaved, Samson finally admits his need for a relationship with God... and God restores his powers. Samson caves in the enemy temple and they all die together.
Nebuchadrezzar.
The Bible, The Book of Daniel, Chapter 4. Nebechadrezzar, the greatest monarch of the Neo-Bablyonian Empire, after admitting God's sovereignty (while ignoring Him), loses his mind. He lives like a beast of the field for seven months/seasons/years at which point his sanity returns and he praises God. He was also the monarch in the story of the lion's den.
Gandalf.
The Lord of the Rings. In The Fellowship, Gandalf falls into a seemingly bottomless pit. He reappears in The Towers. He explains that he died and went back in spirit to the Undying Lands and was refreshed by his angelic leaders. In his new (or at least rejuvenated) body, he finds his powers are magnified or even that he has assumed the powers of Saruman. After fulfilling his mission to save the free peoples, he journeys back to the Undying Lands.
Belgarath. The Belgariad. In
Magician's Gambit, the third book, Belgarath over extends himself and fall unconscious (slips into coma?). During the fourth book, Polgara and Garion discuss the fact that Belgarath's powers may be gone.... but they also decide not to tell him. They are worried that if he doubts his powers, he'll fail when he tries to use them... which will lead to his death. In
Enchanter's End Game, the fifth book, Belgarath tries his first spell in a year... an overly complex and powerful spell that may be beyond his abilities... and he succeeds.
Bayaz. The First Law Trilogy. In
Before They Are Hanged, Bayaz (the First of the Magi) has a spell misfire/implode and he falls into a coma. He eventually comes out after a month or so... and he goes on to cast the ultimate spell and defeat the Hundred in the heart of the Union.
Samson, Nebuchadrezzar, and Gandalf all have spiritual/moral awakenings/blessings associated with the restoration of their powers. In the cases of Belgarath and Bayaz, their powers return after extended healing... much like a broken bone.