Is it my ballast or bulb that is broken?! Please Help!

I have an HTG Supply 400w setup with the magnetic ballast. Yesterday, i went to check on the plants after the timer had kicked on and when I opened the tent there was no light! :( The ballast is buzzing louder than normal and the light won't come on. Does that mean the ballast is out or the light is out? This could be detrimental as it is the weekend and HTGS won't be open again until Monday.

go to lowes or home depot and buy another bulb [you will need it later anyway] if it doesn't light up its ur ballast.2 or 3 days of dark won't hurt ur plants though it may delay harvest slightly.