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The IP Address on the local network, of either the printer, or of the computer (or both) may have changed.
Check both, see if the settings on your iPhone for printing (however wireless printing is configured on your iPhone) match the current IP Address values for (the computer and) the printer.
When in doubt, uninstall the printer on your iPhone 6, and restart the iPhone. Then, after restarting, re-install the printer on your iPhone 6.