which MUST be addressed by the Apple Store.
Unless you purchased your iMac before today's date in 2012, it is still under warranty.
1. Go to the Apple Store and get them to fix it. They may have to send it off for repair
2. Call Apple NOW, and report the problem. They will arrange for it to be shipped somewhere
Let us know how you get on
I cannot restart my iMac (bought in 2012) after having been switched off. There is a horn sound (SOS in Morse code). What can I do ?