The machine powers on, fans spin, and the screen stays black. This is among the most common first-build outcomes and it is usually something simple. Work through these in order rather than changing several things at once.
Check the obvious first
Is the display connected to the graphics card? If you have a card fitted, the cable must go to the card, not to the motherboard. This is the single most common cause.
Is the display on the right input? Monitors do not always switch automatically.
Does the processor have integrated graphics? If there is no card and the processor has none, there will never be a picture. AMD and Intel processors with an F suffix generally have no integrated graphics.
Then the power connections
The 8-pin CPU power connector. Near the top of the board, easily missed entirely. The machine will spin fans without it and never post. Check it is fully seated, not just inserted.
The 24-pin motherboard connector. Push until the clip latches. A partially seated 24-pin produces exactly these symptoms.
Graphics card power. Fully seated, and for a 12V-2×6 connector, pushed until it clicks.
Then re-seat
Memory. Remove all modules and fit one, in the slot the manual specifies for a single module. If it starts, add the others one at a time. Memory not fully seated is extremely common because it needs more force than expected.
Graphics card. Remove and refit until the retention clip clicks.
Between each change, try starting. Changing several things at once means not knowing which one mattered.
Use what the board tells you
Many boards have four small diagnostic LEDs labelled CPU, DRAM, VGA and BOOT. Whichever stays lit indicates where the startup sequence stopped, which narrows the problem immediately.
Some boards have a two-digit code display, and the manual lists what each code means. Both are considerably faster than guessing.
If none of that works
Clear CMOS, which resets firmware settings to defaults. There is usually a jumper or a button, and removing the coin cell for a few minutes with the power disconnected also works.
Strip to minimum: board, processor, cooler, one memory module, power supply. No drives, no card if the processor has integrated graphics, nothing else. If it posts, add components back one at a time.
Check for a standoff under the board where there is no mounting hole, which shorts the board against the case.
If a firmware update is needed for your processor and the board has flashback, this is the point to use it.
Be patient on the first start
A new build, or one with new memory, retrains its memory on the first start. This can take thirty seconds or more with a completely blank screen and no activity.
On DDR5 systems it is frequently longer, sometimes well over a minute, and it may restart itself several times during the process. That is normal behaviour and not a fault.
People reset the machine during this, which restarts the training, and conclude the board is dead. Give a new build at least two minutes before intervening.
Narrowing it down with parts you have
If a diagnostic LED points at the memory, swap modules between slots. If a specific module fails everywhere, it is the module; if a specific slot fails with every module, it is the slot.
If the LED points at the graphics card and your processor has integrated graphics, remove the card and connect to the motherboard. A machine that starts this way isolates the fault to the card or the slot.
If nothing at all happens when you press the button, short the power switch pins on the board directly with a screwdriver. If it starts that way, the case power button or its cable is the fault, not the machine.
When to stop and ask for help
If you have worked through everything above and a stripped configuration still shows nothing, further component swapping without spares has limited value.
At that point the useful information for a retailer or a forum is what you have already eliminated: which diagnostic LED is lit, what happens with a single memory module, whether integrated graphics behave differently, and whether the board starts outside the case.
A build that has never posted is also still within its return window, which is worth remembering before spending a weekend on it.