If it's 3 years old then I don't think it's under warranty anyway, unless you have been paying for extended warranty, and I've never seen much point in that. You don't have to pay too long for the extended warranty before you could have bought a new printer.
My z3100 gets a lot of use, and I still have ink carts that "expire." I've never figured out what this is for except maybe a way HP figures they can sell more ink. I've printed with numerous carts that have expired in use, and it seems now that there is always at least one or two or three carts in the printer at any one time that are expired. I just keep on printing. Never been a problem.
Since you left it turned on you shouldn't have any problem if the power outage didn't result in the printer being off for very long. I would probably just fire it up, re-seat the inks and give it a go. If any carts present a problem, just replace those carts. Good move leaving the printer plugged in and powered on. That should have kept the ink moving through the line as the printer does a few self-checks daily. That is the saving grace of the z printers: no clogs.