How to recover / Clean an HP PageWide printer after extended disuse 

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I just don't understand why after 12h the optimal is reached? What could happen in between this time when printer is in sleep mode that can do the magic work?

This doesn't help when one of the nozzles are clogged and won't print a colour.

Because of covid our printer wasn't used for over a year and now it doesn't print using black ink. We installed new ink and it doesn't print any black