These in-wall devices stop working and need to be replaced from time to time.
There are no repair parts.
Generally dimmers don't last as long as ordinary light switch, because they are electronic instead of mechanical.
Lutron makes the widest selection of dimmers.
http://www.lutron.com/Products/StandAloneControls/Dimmers-Switches/Pages/DimmersSwitches.aspxUse link above to identify dimmer.
For example Skylark dimmer.
Then google search for a replacement.
For example: single pole 600 watt lutron skylark dimmer
Or: 3-way 600 watt skylark dimmer
Lutron also makes 1800 watt dimmers for larger loads.
Add a comment any time if you need more help.
You can also take advantage of fixya phone service.
Fixya expert will talk with you via phone for a price.