I need to buy my favourite

SOURCE: can't get my croton watch to work.

There are a few possible reasons why your watch is not working:

1. The battery is dead (sometimes even new watches have a dead battery in them).
2. The plastic piece between the crown and the watch case was not removed (this protects the battery from wearing down until you receive the watch).
3. You bought a watch with a damaged movement and it just will not work period so you may have to return it, exchange it, or have it repaired

SOURCE: I have lost the back of a gold ladies Geneve watch

Best bet here is to go to a dealer that carries that brand and seeing if they can order you a new caseback.
Or, you can look for the same model online or EBAY that is broken or in disrepair, then just scrap it for the caseback. (It is not always possible to find one, but i recommend trying as this is the least expensive way to obtain small parts).

SOURCE: My DKYN is a ladies

sounds like a broken spring bar.They break all the time and can be replaced by any good jewellery store while you wait.