optiplex 7010 won't start

Have you tried to reinstall the old drive just to see if the system will boot? If it does then possibly one or more of the folders you moved was a Windows critical folder. Are you seeing any error display at all? Have you tried to run diagnostics, F12 at boot, just to confirm hardware integrity? 

This web page at Whatsabyte has some information. Also this Dell support page may help. And lastly this page at the MS forum. One suggestion is that the MBR may be corrupt or missing which might fit considering you moved folders around. In the end it may be necessary to clean install Windows.

As I mentioned, nothing works. The PC starts and all it does is blow the fans for a couple seconds, then switches off and restarts, blow the fans for some seconds etc etc. The monitor doesn't go off sleep mode. I tried with other graphics cards, even without one, nothing changed.

Replace the size 2032 CMOS battery if it's 5+ years old.

If still no luck:  How to Run a Power Supply Unit Self Test On a Dell Desktop or All-in-One Computer tests internal components.  With all external peripherals disconnected, if you push the test button on the PSU, (with PC off) does it give you the green light?  Then internal peripherals are good.  Also:  Dell BIST Test video 

I did the PSU test. When I press the button the fan operates in great speed and the light turns green immediately. As soon as I leave it it turns off and the fan stops. Is this how it's supposed to work?

Other than that, nothing has changed. 

A bad PSU can still start the fans with the test button pushed.  That's what this is looking like.  I take it due to the restarts, you're unable to access the F12 Boot & Diagnostics menu.

One thing to try is removing the SSD, try to start it and see what it does, if power stays on and gives you "No bootable device found."  If it does, turn off, start again, hit F12 immediately and repeatedly upon startup for Boot & Diagnostics.  Arrow down and run Diagnostics.  If that all works well, it's not the PSU and it's the boot drive.  If still no reliable startup, it's likely the PSU.

By the way, what folders were moved?  Was it just files and not OS system folders?

The folders' location that were changed and not moved were documents, pictures, videos, music and downloads. The pc doesn't show anything, as the screen doesn't even become active. The battery was changed and CMOS was cleared, all to no result.    

By any chance did you try it with the boot drive disconnected as described above?

Yes i did that, as i also disconnected every external card connected (serial and parallel card, firewire card and PCI-e VGA card) and still nothing. I don't get any beeps indicating fault on CPU or BIOS. When i remove all memory dimms, it beeps their absence. 

"When i remove all memory dimms, it beeps their absence."  And it stays powered on?  If it does, that's where the problem lies.  If not, I still suspect the PSU.

Yes, it stays on. Every PC i had tested in the past stays on beeping memory absence. What do you mean "If it does, that's where the problem lies"? 

After all of the troubleshooting trying to get it to stay on, if the PC stays on with RAM removed, then it could likely be a RAM problem.  Test one stick at a time in it to see what it does.  Might luck out and it not  be the PSU as RAM is easier to replace.

I almost forgot you mentioned in your opening post that you removed the RAM.  But then it was said that everything was disconnected and the result was the same.  So the RAM wasn't suspected again until more recently.

I tried each and everyone of the memory DIMMs on each and every slot available in the MB (4, 4GB DIMMs X 4 slots = 16 tryouts). None of them had a result, so i have to sadly say that the MB must be dead. Thanks for all the replies.