OS won't boot. Black screen. How to determine the malfunction?

Question

IDE

Bonjour guys. I'm trying to deal with an old motherboard.
ASUS PSL MX.
Problem: after the boot screen, the BIOS crawls out "_" blinks a couple of times on the first line and jumps to the 3rd. Then it flashes endlessly.
Not a single OS is loaded either from hard or from yusb.
I checked all the capacitors on the motherboard (visually everything is in order), put a new battery, tried to put another bar of RAM, disconnected / connected all the usb, picked the BIOS settings.
All to no avail. Tell me how to determine what the problem is?
Characteristics of the "killer machine"
Amibios: v. 1004
Proc: Intel Pentium Dual CPU E2160 (bios sees, I think working)
System memory: 512mb (100% working)
HDD: WD 500gb (100% working)
USB with Linux installed. (Working 100%)
PSU: working 100% works fine with another motherboard.
Bios configs are standard:
MAIN:
Time date is correct. It doesn't crash on reboot.
Third IDE master/slave: WD (hard)
Fourth IDE: DRW (disk drive)
——-IDE CONFIG
Onboard IDE Operation Mode (enchantced mode)
Enhanced mode support On S-ATA
Time out ide detect (35)
ADVANCED
JumperFree config: auto
USB function: Enabled
USB echi hand-off: disabled ----
chipset
CONFIG DRAM timing by SPD: enabled
HyperPath3, DRAM Throtting threshold: auto
Boot Graphics Adapter priority: Internal VGA
Graphics mode: enabled 8mb
Graphics memory type: auto (all)
——
HD audio: enabled
Pciex GbE lan: enabled
Serial port address 378
Port1 address 3F8/IR
Paralle port mode ECP
IRQ7
——PCI/PnP
PnP no
PCI latency time 64
. ..
POWER
Suspend mode: auto
ACPI support: enabled

Answer 1

Reset BIOS.
Check that I saw a flash drive - in the settings from where to boot, its mention should appear.
You may need to explicitly specify what you need to boot from a flash drive

Answer 2

It looks like your OS is not loading. Do you have a boot disk/flash drive? Quickly, you can download a boot image of a Linux thread to a USB flash drive ( Ubuntu is the easiest ) and try to boot from the USB flash drive. If the installation starts, then something happened to the operating system.
Again, it's not entirely clear why the hard drive with the OS is third in the BIOS (Third IDE master/slave: WD)? The disk from which the OS is loaded should be the first in the settings (but this is so, the lyrics, it should not affect the boot).