You're an iMac user because you demand the most power out of your machine.
But now you're not getting any juice, which means you're not using any of that potential.
Replacing your power supply with a working one may help rejuvenate your iMac to its full power.
Please check your PC's Model and Year before purchasing, thanks!

Power: 300W
Input Voltage: 100V - 240V / 4A, 50/60Hz
Output: +12Vs5/ 25A; +12Vs 5 DC/ 5A
Apple Part #: PA-1311-2A
Printed Part #: ADP-300AF

A list of Compatible with iMac 27" A1419

iMac13,2 Late 2012: MD095LL/A (2.9 GHz Core i5)
iMac13,2 Late 2012: MD096LL/A (3.2 GHz Core i5)
iMac13,2 Late 2012: MD096LL/A (3.4 GHz Core i7)
iMac13,2 Late 2012: MD095LL/A (2.9 GHz Core i5)
iMac13,2 Late 2012: MD096LL/A (3.2 GHz Core i5)
iMac13,2 Late 2012: MD096LL/A (3.4 GHz Core i7)
iMac14,2 Late 2013: ME088LL/A (3.2 GHz Core i5)
iMac14,2 Late 2013: ME089LL/A (3.4 GHz Core i5)
iMac14,2 Late 2013: ME089LL/A (3.5 GHz Core i7)
iMac15,1 Retina 5K Late 2014: MF886LL/A (3.5 GHz Core i5)
iMac15,1 Retina 5K Late 2014: MF886LL/A (4.0 GHz Core i7)
iMac15,1 Retina 5K Mid 2015: MF885LL/A (3.3 GHz Core i5)
iMac18,3 Retina 5K Mid 2017: MNE92LL/A (3.4 GHz Core i5)
iMac18,3 Retina 5K Mid 2017: MNEA2LL/A (3.5 GHz Core i5)
iMac18,3 Retina 5K Mid 2017: MNED2LL/A (3.8 GHz Core i5)
iMac18,3 Retina 5K Mid 2017: MNED2LL/A (4.2 GHz Core i7)

Package Contents:
1 x Power Supply for A1419 + Warranty 6-Months