Normally, most people will restore iPhone from iTunes. But that is not what we need to talk about here. Here we focus on the scenarios where iTunes is not working, and they are listed below:
1) iPhone is disabled or gets stuck in recovery mode:
When your iPhone is disabled or stuck in recovery mode / DFU mode, it is obvious the recovery mode cannot function normally, which is the prerequisite to restoring from iTunes. And the natural result is that iTunes cannot restore your phone.
2) iTunes errors occur:
When you try to restore your iPhone, a notification of iTunes error pops out, informing iTunes fails to restore your iPhone due to this error. In this case, iTunes will not be able to restore your iPhone until you resolve the iTunes errors.
3) You want to restore your iPhone but don't want to hurt the data:
It is known that iTunes will erase all your precious personal data while restoring your iPhone, which is certainly undesirable. Therefore, when you just want to fix some trivial problems but still want your data to remain intact, iTunes shouldn't be the tool.
4) You have just got the iPhone and not yet authorized it:
A small group of users is so lucky that their iPhone got issues not long after they received their iPhone. This happens so fast that they hardly have the time to authorize the device, and iTunes won't be able to restore this phone.