Jim T's sworn enemy!
If the zippers are in fact plastic then they may not be stuck but jammed by the teeth(had that happen a few times on my enclosure)Just got to keep working it to get them to pop loose. Worst case is to pull one tooth off,slide the zipper half out and start over again from the beginning and have the tooth section repaired. This is usually caused by worn zipper teeth. If the zipper is aluminum(most are)then its oxidized. Tap it with a screwdriver handle several times to break the oxidation loose and it should start working again. On my enclosure I treat the zipper teeth with a petroleum(sp.)jelly like vasolene(again sp.) and haven't had a problem since.