I would reset the SMC, unplug the iMac hold the power button for 10-20 seconds (unplugged) then Start the iMac as usual. https://support.apple.com/en-us/HT201295
An upgrade to a 7200 RPM drive would be no-problem, Just be sure to purchase a fan control software because apple's Hard Drives have special temperature probes.**iMac Intel 21.5" EMC 2428 Hard Drive Replacement
i had this issue as well and i figured out that the wall to the right of the power supply was making it difficult to get the logic board back in place; i removed that plastic wall and made sure no cables were blocking the board and it went back in no problem.
i had this issue as well and i figured out that the wall to the right of the power supply was making it difficult to get the logic board back in place; i removed that plastic wall and made sure no cables were blocking the board and it went back in no problem.