A surprising number of day-to-day problems are solved when you realize that the thing just isn’t plugged in. Oops.
The obvious sense of “plugged in” is “plugged into the mains supply” - the AC wall outlet.
But there are many other useful senses to look for:
Are other cables (not power) plugged in between two things? Are the cables themselves working? Network cables, speaker cables, video cables, printer cables…
Are software components finding each other correctly? Mail clients pointing to the right mail server, DLLs found and loaded, CGI URLs correct, variable names spelled right…
Are people communicating - “plugged in” to the project, both logistically (e-mail address or phone number correct) and mentally (same expectations on both ends)? No news is not good news…
What must connect?