Once i get the water chip i take the elevator down to get out from the vault...i get out and there is a ladder which u need to take and every time i climb it it will put me back on the same screen..cant move from there...can anybody tell me why?

If the problem persists, try to re-load your game. Or load a save before you entered the vault.
I had a problem when my character was jumping on the ladder, instead of climbing.

Just Extra links to help out on the quest:-------
https://www.youtube.com/watch?v=Uu7Q6q5o8sE
http://fallout.wikia.com/wiki/Find_the_Water_Chip
http://fallout.wikia.com/wiki/Vault_12
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Hope this helps
gogwitcher300