As you describe it, a new motherboard (assuming the CPU and RAM aren't defective) and video card should have helped, because they would show at least the BIOS messages before Windows starts to load. A monitor showing sleep mode means it doesn't get any signal.
I assume you already tried another monitor. Those are much easier to swap than a motherboard.
But maybe your description isn't fully right, and it only goes into sleep mode after Windows starts.
1. Can you go into the BIOS setup by pressing the right key (I don't know which one, but usually it's shown on the screen and documented in the user manual) very early during the booting?
2. Does it boot from a bootable disk (such as Windows setup, Linux, DBAN, memtest86)? If it boots into memtest86, run it.
3. Does it boot into the Dell recovery partition or the Dell diagnostics? For both, see the user manual. If it boots into the diagnostics, run them.