Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
Lets try this.. Open the printer like you were going to change the ink tanks. If y ou can, remove the ink tanks.. If you cant, leave it open.
Now, unplug the printer from power and press and hold the power button for 30 seconds. Now leave it unplugged for 5 minutes.
Then, hold the power button and plug the printer back in while holding the power button.. Any change?
Dustin S.Senior Technician and Network Switching Systems Operator / Maintainer.
Question.. When you press the power button.. Do any of the lamps on the machine flash or light up?
Also, were you able to remove the ink tanks? If so, lets also remove the printhead because a failed printhead has been known to cause the machine not to power on. ( and since it cut off during printing, the printhead may have over heated and failed )
So remove the ink tanks and the printhead.. And then unplug it and plug it back in and try to power it on.. What happens?
Thats probably our worst case scenario then.
Either the power board or mainboard has failed, and you will need to replace the printer.
Im a fan of the MX922 model which is similar to your Mx860, just a bit newer. ( It has the wireless printing, scanning and smartphone connectivity features )
A final piece of advice, if you are printing a lot of pages in a short time period. Remember these home use printers arent made for mass printing. So my advice is if you print more then 25 pages in a row. Try to print 10 copies.. Wait about 3 minutes, t hen print 10 more.. Etc. This will ensure the printhead has a little time to cool off and not over heat. With that being said, these consumer model printers are usually only useable for 2-4 years before something fails.
If you have any further questions, let me know.
Youre welcome.. If you have any further questions, let me know.