GS752TPS / GS728TPS not showing in stack

After doing a maintenance, which included opening my switches and blowing compressed air on the components, 2 of my switches don't show anymore in the stack. I know that they are both working, but the web interface only shows my main switch and the other 2 are showing as "not present". the Netgear switch discovery tool doesn't see them and i can't access them through their reserved IP.

one of those was previously reporting that it had dead fans, i contacted support to see if i can RMA or get replacement fans, they requested me to reboot it and see if it stops reporting that, but now that i can't even see it anymore, i can't confirm it... it's located in an enclosure in a place that isn't easily reached and i can't just turn it off and on, i need to do this during an off-day.

what can i do? is there a reason that it would have stopped talking to me?

thanks for your help.