How do you fit a copper pipe into a miniature valve ??

Hi all, please forgive my poor knowledge about plumbing, as well as my poor english, but I really need advice here.
I just had a kitchen fitted in my house in a new room.
The pipes under the sink were slowly leaking so I had a look and isolated the link. It happens between a white flexible plastic pipe coming down from the sink into a copper pipe (cold water). I am thinking of unscrewing that junction and of re-tightening it in after adding some watertight "white stuff" (sorry about that, I am referring to that thin white plastic thing you see in rolls).
As I was having a look, the other side of the copper pipe came off from a miniature valve (the one you close/open with a screwdriver) in which it was embedded. It looks really odd to me how easily that pipe came off, it seems there was no solder or any material, the pipe was just slid into the hole. Now I want to fix that, what should I use ?? How do you fit a copper pipe into a miniature valve ??
I know you will all tell me to call the fitters and complain, that I will do, but between now and the 3 weeks they are going to take before coming back, I would like to use my sink, so I'd like to fix it.
Thank you all for your time.

Well. without a picture to show for sure, its hrd to say but I'd suggest getting hold of that 'white' stuff (PTFE tape) wind it around the existing pipe (the bit that fell out) and re make the joint.
Bear in mind that you must use enough but bot too much of this stuff.
As a general rule, twice arount the pipe should be enough.
Try it and let us know how you get on.
garethgas

Yes sorry for the absence of picture.
I will try what you suggested.
many thanks for the help.
D.

Hang on, if a joint "fell apart", it won't be one suitable for ptfe tape. Either it's broken, which wouldn't be surprising on a cheap flexible tap comnnector's isolating valve, or it's Pushfit.
If it is a pushfit, push it back together and pull hard to try to get it off, which should make it grip..