Ran out of ink.

You are good to leave it overnight. We have demo machines that sit for much longer than that without issue. It might require a cleaning cycle to bring back some nozzles that might have dropped out but other than that you're good to just leave it.

 

I also have a 540i. Just cut the tab on it and stick it back in. It still has 5% - 10% of ink still in it. It's enough to clean the machine a couple of times.

EDIT: The tab on the ink I mean

 

If you wouldn't turn it off you could have inserted the new cartridge and continue the job where you left it. When the machine says the cartridge is empty there is still 10% ink. I usually use a large syringe to pull the ink from the 'empty' cartridge and insert it into the new one.

 

As AMS says, either trim the ink level tab on the back of the ink tank or shove a piece of folder paper/card in there to hold it back so the machine still thinks there's ink in it until tomorrow.
It's likely not completely empty.

 

I've just put a piece of masking tape over the tab in a pinch. That will get you through the night.