Start again, it's not too bad disassembling an iMac G4. Ensure that your thermal paste isn't conductive and clean any remnants off with isopropyl alcohol wipes. Just a thin layer of the cheap white silicone goop is all you need on the heatsink "base". Also those PATA to SATA adapters don't work very well either, so perhaps disconnect the SSD when you try to fire it up. Double check all connections too, some need a firm push to go back in.

JB