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It is probably deleted from your dock or moved to some other location. In Finder, click on the search magnifier (Spotlight) at top right corner and type Pages. If the Pages is still on your Mac, then it will find and show. So open it from there. After opening, it will appear on dock. Then right click or hold down Control key and do a click on the Pages icon on dock, select Options and select Keep in Dock. This will add it back there.
If you don't see it there, then look in Trash. If it is there, then right click on it and select Put Back.
If you still can't find it, then you can redownload it. Go to Apple > App Store. Search Pages and then download it.
Please let me know if that didn't work, so we can continue troubleshooting
On your Mac, click on the Magnifier icon at top right and search (type) Pages. Does it show the Pages app ?