pc powers on but no display, beep or anything

First question is: what does it do? Any lights? Fans run? Or is everything dead?

The first guess, based on what you have described, is the power supply is going/gone.
Can you borrow a known working supply to test with?

But if there is nothing happening at all it could be as simple as a case switch.

Still could be the PS, but not so obvious now.

The way to diagnose this problem is to try and isolate the cause. We do this by process of elimination (or addition depending on how you look at it).

Basic diagnostic procedure is to uninstall the entire build and start over.

Some people do this out of the case by placing the motherboard on an anti-static surface. This eliminates the possibility that there is a short circuit between the motherboard and the case through the connection posts (stand-offs). A standoff in the wrong location or a sloppy job will cause a short.
(You probably don't need to do this IF the PC was working fine until recently - this step applies to newly built rigs)

The Test (power off, power cord unplugged):

• Disconnect everything externally connected except the mouse and keyboard (printers, USB devices, etc). If you are not using a wired mouse and keyboard see if you can borrow one. The wireless device is just another component you have to deal with.
• Disconnect the power and data cables from all the drives inside the computer (Hard drives, DVD/CD drives, etc).
• Remove all the cards installed in the expansion slots (PCI/PCI-e) including the video card. (Be careful handling them and place them on a non conductive surface while testing).
• Remove all the RAM sticks (same rules as above).

Now connect the power cord and turn the PC on.

• The motherboard should start beeping. You should get a beep code that tells you there is no memory. This is good, it means the processor is functioning and the motherboard is good so far.
• Now add one stick of memory in Slot A1 and power on. More beeping: "no video card" beep code. This is good.
• Then add the video card and connect it to the monitor. You should get no beeping and you should see the BIOS start screens, ending with the message that there is no boot device.
• If you get no video then switch the one memory stick installed for another one and test.
• If you do get video then start adding components back, one at a time, until the system fails to boot. The last component you added is then the problem component.

Don't deal in "what if".
Deal in "what is".
Testing is necessary. If you are not comfortable inside your computer then find a freind who is, or take it to a repair shop. There really is no other (easy) way.

EDIT: forgot to answer your question.
If you take it all apart and there is only the motherboard, processor, CPU fan, and power supply connected, and it does not beep, then you know the problem lies in one of those 4 components.