ALL interface icons are gone?
Don't panic. :)

1) Try relaunching Finder: 


  You can do this by logging out and back in, or by restarting the system, or more quickly by force-quitting the Finder using the system's Force Quit option. Press Option-Command-Escape or select "Force Quit" from the Apple menu, and then select and quit the Finder. The Finder will relaunch automatically, and hopefully show the icons properly loaded.


2) If the problem persists, it's time to clear the caches. There are different ways to accomplish this. The easiest and less error prone is to use a free maintenance utility, like Onyx for Mac, from Titanium Software. Onyx is an old school Macintosh application and will do this (and much more) for you. It's free as in beer and it's very easy to use. 

If you feel like you want to pay for an utility, I have the fantastic Cocktail, which I brought a long time ago and I only use "every now and then" when I have a problem like this. I haven't compared both (or any of the other alternatives) but they usually do the same stuff under different user interfaces. Cocktail releases periodic updates and the UI is clean and nice, but other than that, any utility will do.

Since I don't have Onyx installed right now, I'll show you how you could do this with Cocktail:

In Files -> Caches -> select all like in the sshot:



Then go to Options and select all:



and finally also select all these: 



finally… click ok and in the first screen click Clear.

It will take a while. Restart your computer and tell us what happened :)

(I don't do this clearing very often, but every 6-12 months I usually clear stuff, it shouldn't be needed… under normal circumstances).

Your computer may feel slower and/or sluggish the first couple of minutes after the boot, that's because the Cache's have been deleted and they must be recreated. Eventually it will catch up.