Replaced hard drive. Fans super loud and no display

After several months of super slow performance and pinwheeling with my iMac I started to suspect that the mechanical hard drive was slowly failing so I attempted a swap with a SSD. I purchased an adhesive and cutting wheel kit and the display came off no problem. I then swapped out the drives with no issues either. At that point I reassembled with painters tape and powered the machine on. The display remained blank and the fans are spinning at full speed. LEDs 1, 2, and 3 are light and the 4th is off. I also tried an external monitor connected via a mini displayport to displayport cable but I’m not getting video there either.

At this point I’m not sure where to go next. Can anyone help?