Hello
Thank you for posting in the HP Support Forum. 
I understand you're encountering a recurring error on your HP Photosmart 7520 e-All-in-One Printer. I would like to assist you with this.
If you click here you'll see that short of removing and reinserting the cartridges, cleaning the printhead and doing a power reset (unplugging print for one minute and rebooting), there's no other official recommendation from HP to clear out the error message.
If you have tried all of the above and your issue persists, click here to check your printer's warranty status. If your printer is currently under warranty, I recommend you reach out to our tech support team. You can click on the following link to initiate a case: www.hp.com/contacthp/
Once you fill out the online form to create your case you can choose from different direct contact methods.
If your printer is NOT IN WARRANTY, try the following:
- With your printer powered on, open the cartridge access door.
- Once opened the printhead will move to the centre - take a damp cloth or q-tip (moistened with bottled or distilled water) and carefully clean the underside of the printhead. Do not allow water to drip into the inner workings of your printer.
Let the printer sit overnight (longer if possible) and try printing again. If you still cannot print in black reach out to to technical support as directed above in spite of your lack of warranty to look into alternative service options. Please let me know the result of your troubleshooting by responding to this post. If I have helped you resolve the issue, feel free to give me a virtual high-five by clicking the 'Thumbs Up' icon below and clicking to accept this solution.
Thank you for posting in the HP Support Forum. Have a great day!