Print a test page (Start > Printers and Faxes ... right-click your printer icon, select Properties ... click the Print Test Page button).
If the problem is your black cartridge, it'll be obvious on the test page.
Yesterday, we were having the same problem however the test page printed perfectly. Today is a new story. The test page now just printed the 4 color Windows flag thing and nothing else. Strange thing is the Windows flag was not on yesterday's test page.