Hi waleed444,
please click on the Start button to go to the start/Metro screen, take your pointer to the lower left corner of your screen to open the Charms Menu, click on Search, you will see all the applications listed in alphabetical order, find and right
click the Weather app, click on the Pin button that appears on the lower right of the screen. Hope the Weather app is pinned back to your start screen.
If that doesn't help you may Restore your computer to an earlier time. Search 'Restore' in Settings, restore your computer to a date when your
Weather app was present. You may have to reinstall the programs/apps you have installed after the selected date.
Irfan