HP L26500 not detecting printhead

I have a problem with my black/yellow prinhead no 3. So I've switched the no3 and no4 because sometime it correct the problem. But now, no matter what I do, the printer don't ''see'' the printhead no4. Even if I switch them back or use an old printhead. I've clean the connector with the tool and tampon that came with the printer but still don't work. And now, I can't shut off or reboot the printer (I did not use the paper clip hole yet).

What should I do then?

Thanks.

 

So I've done all the electrical test, clean up the connectors and you know what? It was just that the connectors and printhead was not perfectly aligned. I've put my finger on top of the connector strip when installing the printhead and the printer was able to read the printhead. (I hope I'm clear enough).

I'm typing this in case someone else would have a similar problem and make a search for it.