A toilet tank (not the bowl) that won't fill after flushing and/or continuously runs is usually caused by a worn or improperly placed "flapper" (i.e., the seal or valve that covers the water outlet tube at the bottom of the tank; n.b., some units may have a plastic valve). Sometimes, jiggling the handle will cause the chain or lever to re-seat the valve, correctly, but this can also be done by lifting the tank cover (on most units) and re-seating the valve, manually (n.b., the water is usually very cold, and the valve may be very dirty, especially if it's rubber or neoprene or similar). However, in some cases, the valve is simply worn out; replacement is usually required after five or so years, depending on the type of valve, the condition of the water (i.e., salinity, hardness, etc), and, of course, how often the toilet is used. It is actually very simple to fix a leaky or worn valve, but it is not for someone who has never used tools or is not familiar with how indoor plumbing works, so I won't list it, here. You can get replacement parts at virtually any general retailer or hardware store (make sure that you know which part you need; bring it to the retailer, if possible). Ask for advice on repairing the tank; some retailers have free do-it-yourself sheets, but you can also find tips on the web by entering "fixing a toilet bowl" into your favorite search engine.
