iMac won't boot up

Sounds like a power issue. Did you have a lightning storm or high winds in the area? What can happen is a power surge zaps your system power supply and it can also damage your logic board

Time to inspect things to see if you ca spot the damage. Follow this guide to check the Power Supply iMac Intel 27" EMC 2429 Power Supply Replacement you’ll need to inspect the capacitors and power transistors for swelling or burnt. The main logic board will likewise need to be pulled to inspect it fully as well. Pickup from Step 14 in this guide [Installazione del Kit Doppio Disco nell'iMac Intel EMC 2429 da 27"

Let us know what you discover!