Perhaps the sidebar with the clock just hidden. To display it on the desktop, move the mouse cursor to the bottom right corner of the screen, locate the sidebar icon and double click it. Sidebar clock is displayed again.
If you can't find the sidebar icon in the lower left corner, then it's closed. To open it, click "start" - "All programs" - "Accessories" - "Windows Sidebar". Sidebar clock appeared.
If the sidebar is open and the hours on it, most likely, is a mini-app was closed. Then right-click the sidebar and select "Add a gadget". Find the clock icon and just drag it to the sidebar. You can also double-click on it or click the icon with the right mouse button and choose "Add". The clock displayed on the desktop.
If the clock icon is not on the settings page sidebar - so this is the gadget to be removed. You can always restore the gadgets that come with Windows by default. To do this, right-click on the sidebar and select "Properties". In the appeared settings window, click "Restore gadgets installed with Windows". There is a window of gadgets that are on it the clock icon. To display the clock on the sidebar, just drag them there or double click on the gadget icon.
You can install other, unusual hours, which are not included in the Windows installation package. Right click on sidebar and select Add gadget. In the opened window, click on the link "Find gadgets on the Internet". A page with a gallery of Windows gadgets.
At the top of the page in the search bar, type "Watch" and click "Search". Page shows you different types of watches. They can be digital, analog, speaking, etc., Choose those that you liked most. Click "Download" - "Install" - Save. In the opened Windows Explorer, select a folder to save downloaded widget and press "Save".
Now go to the folder, double-click on the gadget icon and click "Install". Mini-app is installed and added to the sidebar, and the new clock is displayed on the desktop. You can always close this app and install another.