We all know that Thor's enchanted hammer, Mjolnir, can only be lifted by those who are "worthy." Over time, we have seen the God of Thunder lose his worthiness - and his own self-worth - plenty of times. But in Jason Aaron's 2014 Thor run, Thor lost more than his worthiness - he also lost his arm.
Thor #1, titled "If He Be Worthy," opens with a sort of prologue, which reads:
"Thor, the God of Thunder, now finds himself no longer worthy of wielding Mjolnir…In a recent battle on the moon, Superspy Nick Fury— empowered with secrets he stole from The Watcher— whispered something that caused Thor to drop Mjolnir to the Moon’s surface, where it has remained ever since. No matter how hard he tries, Thor cannot lift it.”
Even Odin Allfather himself is shown to be unable to pick up the hammer, so no dice on just removing the enchantment. After spending many moons on the moon trying to lift Mjolnir with no avail, Thor gives up and goes to retrieve a new weapon- an axe called Jarnbjorn.
With Jarnbjorn, Thor confronts the dark elf Malekith, who fans may remember as the antagonist of Thor: The Dark World in the MCU. Malekith once again has his sights set on Earth (or Midgard, as Thor would call it) and Thor aims to stop him. Unfortunately, Thor is nowhere near as powerful without Mjolnir. Malekith easily defeats the hammerless God of Thunder, then sadistically jokes, "without that big heavy Mjolnir to lug about all the time, you’ve no more need for so many cumbersome arms.” With that, Malekith takes Jarnbjorn and uses it to cut off Thor’s left arm.
Fortunately for Thor, his lost arm is quickly replaced. In Thor #4, Thor is shown to have survived his encounter with Malekith and has been brought back to Asgard, which has been renamed Asgardia. Odin states that he will not have the heir to his throne "fumbling about like a common cripple," so he has the dwarf Screwbeard fashion Thor a new arm - one made of black Uru metal and forged in the same fires as Mjolnir was centuries ago. Screwbeard says that the new arm is basically indestructible, meaning its actually an improvement from Thor's old arm. The dwarf sure knows how to look on the bright side.
It will be interesting to see just how much the upcoming Thor: Love and Thunder will borrow from this storyline. It's already been confirmed that Jane Foster will become a female version of Thor just as she does in Jason Aaron's series, so there's a chance Hemsworth's Thor will wind up losing an arm, too. At least that would be something for him and Bucky Barnes to bond over. But we also know that the MCU's Thor is currently off with the Guardians of the Galaxy, so if he does wind up needing a prosthetic arm, he had best keep an eye on Rocket.