1. If there is a hexagonal, probably plastic, piece remaining on the shower cartridge, then it may be that the screw which hold the handle on has simply come undone. Phot id possible.
2. I doubt you'll get that spring as a spare part, unless you know the make and model of the shower and are prepared to buy a complete new handle.
3. See if the little spigot which slides in the slot unscrews from the override button shaft (needle nose pliers job). If it does, you could measure the hole and perhaps get a spring off Ebay or similar. Ideally it would be bronze or stainless steel, but bronze springs are very hard to find, and stainless isn't generally a spring making material.