I left my Epson 9900 unplugged for 2.5 years.
restarted and 8 of 10 heads work perfectly after a few cleaning cycles.
the other 2 heads, which happen to be a specific pair, are not working at all. Not one nozzle.. So I'm wondering if there's a problem in the valving/damper assembly. I'm going to have to make some time to take the thing apart and find out if the ink is even coming to the heads for that pair. I can hear the piezos activating during cleaning cycle for that pair but nothing coming out after the first test.
One thing to try is to take a strip of paper towel that is dampened with non ammonia windex ahd slide the head over it, leave it in place overnight. If the clog is in the nozzles, it will dissolve the ink.
If its in the dampers, there is little to do, but you can put a ink tank that is filled with head cleaner for the offending color nozzles and print to just that one clogged color. There are patterns available for each color in the printer, so you can work on one color without using other colors of ink up. It takes lots of paper, but ordinary inkjet paper works fine for this. I've done both before using the windex, I keep going until the faint blue windex is printing a good pattern with no blocked nozzles, then put the color ink in place and print that color again, 25 pages at a time until the ink is printing clean and all the windex is out of the lines and head. It takes a while, but it worked for me.
Leaving my printer off is much better than leaving it on.