1) Electronics inside dimmer can go bad, and lights will not work as before.
Simply replace dimmer.
I buy new dimmers at Home Depot.
2) Lutron shows a wide variety of dimmers:
Use pictures from page to identify type of dimmer you want, and then google search: For example search for 'skylark dimmer' or 'rotary dimmer'
http://www.lutron.com/Products/StandAloneControls/Dimmers-Switches/Pages/DimmersSwitches.aspx3) Add a comment for more free help. For example how to install new dimmer without calling electrician.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on dimmer, or any do-it-yourself project.