Repairs

I found Health Studded Bracelet with a 0/0 durability. I cannot find a single NPC willing to repair it.

0/0 is considered completely broken and cannot be repaired at all. Unless you want to use a Golden/Platinum Hammer of Durability, it can not be equipped until one of the two hammers is used.

When an item is repaired, there is a small chance that it will permanently lose durability. While the max durability is expressed as "#/X," X can be permanently lost, while # is just durability loss from being worn. Once X is 0, it can no longer be used by any means. While some items are an exception to this rule, most equipment are rendered useless once the maximum durability is 0. You won't be able to restore the Durability unless one of the two hammers I listed above is used. Basically, there's broken, then there's permanent breakage.

Also, There are ways to reduce the chance of breaking durability permanently, but for a 100% prevention of lossing durability during repairs, the Repair Protection Potion can be used, something limited to the Item Shop and is quite expensive for many.