Can you be more explicit about what you mean by being unable to print?
1. Are you getting any error messages?
2. Is the printer running paper through but not printing on it? That's a sure sign of dried up ink in the print head. Notorious of seldom used ink-jet printers.
3. Does the printer appear in the Print & FAX pref pane?
4. Are there any lights on the printer? What might they be and what do they indicate?
5. Does the printer make any noises when turned ON/OFF?
6. Have you tried a different ISB (I assume) cable?
7. Is the printer selected in the Print dialog?
Opinion: If all you print are invoices, especially black and white, there's no point in wasting money on ink carts. Laser printers are cheaper now than what ink jets cost a few years ago. Toner is already dry, so that will never be a problem. And the fixed toner is more water proof than the paper it's on!
A very usable, fast, multi-function laser (printer/scanner/copier/FAX) can be had for around $200-$250. And the per page costs will also be lower while looking more professional.
Others may have other opinions, but I never listen to them!