At best, the black ink cartridge is empty - although the indicator guage in the software may show a full cartridge, this is probably not the case. This guage is reset to full whenever the Epson utility software is reloaded.
Buy a new cartridge - genuine Epson for your model is $35.00, or a "clone" brand. I have used Tonny from TEG Computers for the past 2 years on my Epson 760 and very happy with them. A "clone" will cost you only $5.60.
At worst, the printer head is damaged/blocked. If using the supplied head cleaning utility does not work, then buy a new printer; the repair bill will be around $100-$150.
Look at a Canon i350. Very cheap at around $120, very fast - faster than your 640, and ink cartridges (even genuine) are cheap.
