my imac will not turn on? what could be the problem?

I have a 2011 iMac.  Two days ago it would not turn on...have checked the power outlet and it is working.
I suspect the problem is not in the cable to the power bar.  This machine was a lot of money and was working
well 3 days ago.  Do you think it is the power supply or could it be worse and be the motherboard?
My question is related to a major concern...I do not want to be raked over the coals by some apple service centre
to run their diagnostics and levy a very large cost in $.  What are the chances that it is the power supply?  If it
is just the power supply I am prepared to pay the $ for the power supply but if they try to take me around the bend I will dispose of this computer.  What is your advice?
Richard ChristensenRetired computer technicianAsked:
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JohnBusiness Consultant (Owner)Commented:
I think the power supply is the likely candidate. You need an Apple power supply. Look for an independent Apple service store and ask their cost for an estimate of repairs. Then decide whether you should replace the machine.
Gerwin Jansen, EE MVETopic Advisor Commented:
What happened between when it worked and when it stopped working? Did you have a power failure? Did you switch it off normally? What is the exact model you have?

A power supply is about $40-50 on Ebay but replacing it is a bit of work and you would need some tools (see: link).
If you are confident is your handy skills you can do a basic diagnose your self. Many videos on youtube will show you how to open iMac (and tools are not that special, you can always improvise if you don't have them). First smell it the back of the screen, near the cable plug. Usual power supply issue is burned component.
The second check is plugging the iMac into the LAN and scan the network from another device. You might get ping response.
The third check will be removing the screen and power the iMac. Check if there is any led light working on the motherboard, is the CPU fan spinning, is the HDD spinning (ignore this if you have SSD).
If all of this is not working, you can start the power supply removal. Ask someone to test the power supply with a voltmeter before you buy a new one because it will be a waste if the problem exists with the motherboard or CPU.

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Richard ChristensenRetired computer technicianAuthor Commented:
Thanks Nick.  That is a very good answer.
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