If you have a problem with any of your Yahoo! services, the best way to resolve it (as others have said on this thread) is to find the help page on their website and choose the right option.
Most of Yahoo! support at this point as automated and it’s very difficult to reach a live human at their Sunnyvale corporate headquarters. I am assuming you have a complaint and simply want to get satisfaction? That being said, the best phone number is their corporate office at 408-349-3300.
If you would like to write a letter to CEO Marrisa Mayer you may mail her at 701 First Avenue, Sunnyvale, CA 94089. Although it’s highly unlikely this will get the results you are looking for either. Still it’s worth a try. The main HQ for Yahoo! is their Sunnyvalle office. Their investor relations (IR) number 408-349-3382.
There are also a few more options for contacting Yahoo! including social media. They are fairly active on Twitter, which I have added below as well. Take a look at their social media outlets including Facebook at Yahoo Customer Care and Twitter at @YahooCare.
Random Yahoo! Fact: the company was founded in 1994 by Jerry Yang and David Filo. The acronym stands for Yet Another Hierarchically Organized Oracle.