After SSD replacement, no chime or turning on at all

I recently replaced the fusion drive in my 2017 5k iMac with an SSD, following the iFixit guide. I’ve done this previously on a 2012 21.5 iMac which went off without a hitch.

Unfortunately, after putting the screen back on, there’s no chime, no boot - zilch. Any ideas as to where to start? During the process, nothing was bumped, I didn’t move any logic board parts (just the left speaker slightly to the left to get to the 3.5 HDD bracket).