I take it that your slider won't connect the two sides of the zipper but just slides along.
This is a common problem and is caused by the slider having opened up a bit.
Get a pair of pliers and gently squeeze the slider front to back so it'stighter on the zipper, this should do the trick.
(Check the zipper for a cracks first, if it's cracked this will be a very temporary fix.) There is acompany that makes a replacement slider that can be fitted to an existing zipper called ZlideOn
www.zlideon.se
I carry a pair of these in my backpack, one latrge size for the tent and one that fits my jacket/sleeping bag/inner tent.
Ingenious product!