And it is obligatory to resume the I'tikaf, because he failed to perform what he vowed according to its required state.

532 - Issue; He said: (And if a fitna [discord/trial] occurs from which he fears, he may abandon his I'tikaf; then, when he is secure, he may build upon what has passed, if he had vowed specific days, and he shall make up for what he missed, and offer an expiation of an oath [kaffarat yamin]. This is likewise the case in the event of a mobilization [nafir] when there is a need for him.)

The gist of this is that if a fitna occurs from which he fears for himself if he remains in the mosque, or for his wealth due to looting or fire, he is permitted to abandon the I'tikaf and exit. This is because this is a situation for which Allah the Almighty has permitted, by the basis of the Sharia, the abandonment of an obligatory act—namely, Friday and congregational prayers—so it is more fitting that it be permitted for him to abandon what he made obligatory upon himself. Likewise, if it becomes impossible for him to remain in the mosque due to an illness with which he cannot stay, such as continuous standing, urinary incontinence, fainting, or if he cannot remain except with severe hardship, such as needing assistance or bedding, he is permitted to exit. If the illness is mild, such as a headache, toothache, or the like, he is not permitted to exit; and if he does exit, his I'tikaf is nullified. He is also permitted to exit for that which is incumbent upon him by way of obligation, such as exiting for a mobilization [nafir] when it is general, or if an enemy arrives whose ferocity is feared, and there is a need for the person in I'tikaf to go out; in this case, he is obligated to go out because it is an incumbent obligation, so he is required to exit for it, just like exiting for the Friday prayer. If he exits and then his excuse ceases, we must look: if it was voluntary, he has a choice; if he wishes, he may return to his place of I'tikaf, and if he wishes, he may not return. If it was obligatory, he returns to his place of I'tikaf and builds upon what has already passed of his I'tikaf. Furthermore, the vow is not devoid of three states: The first is that he vowed I'tikaf for days that are neither consecutive nor specific; for this, no make-up [qada'] is required of him, rather he completes what remains for him, but he starts the day on which he exited from its beginning so that it may be consecutive, and there is no expiation upon him because he performed what he vowed in the correct manner, so no expiation is binding upon him, just as if he had not exited. The second is that he vowed specific days...