Buy some WD40
Silicone spray, spray the entirety of the zips length and fastener itself, inside and out. Try if you can to force the zip back to one corner, if it doesn't open use some pliers to gently ease the assembly apart. Once the zip is where you need it use the pliers at the back of the zip to gently bite into the zip. The silicone spray will do its job and give a nice free running zip. I hope this helps. I use the silcone spray on all my jackets/ruck sacks etc for trouble free use.