Better delete your serial number from here.
Disassembling this model is a bit tricky, but possible ->
https://www.ifixit.com/Device/iMac_Intel_21.5"_EMC_2638
As the iMac is 5 years old, the HDD could get broken as well. If you can, make a backup ASAP to some external drive. Then try to boot up the external drive. If that is always working immediately, it's not the button, but the drive. After having a backup a PRAM reset is worth a try, too.