Where could part of the RAM memory go?

Question

RAM

Hello.
Today I encountered such a problem, half of the RAM memory disappeared right while working on a PC. Physically stands for 3 years 4 bars of 4GB, everything worked fine.
Memtest says that two sticks work as DDR3-1333 and two are marked as no mapping (interleaved device). The BIOS also defines only 8 GB, there is no integrated video.
I did not make any settings in the system before the appearance of this problem, and the system worked normally.
I set the bars separately - they all work, but they all do not want to at the same time.
AIDA sees all 4 bars.

ChannelA-DIMM0
Module name Kingston 9905403-478.A00LF
Serial number BC1D4E64h (1682841020)
Issue date Week 46 / 2012
Module size 4 GB (2 ranks, 8 banks)
Module type Unbuffered DIMM
Memory type DDR3 SDRAM
Memory speed DDR3-1333 (667 MHz) )
Module width 64 bit
Module voltage 1.5 V
Error detection method No
Refresh rate Normal (7.8 us)

ChannelA-DIMM1
Module name Kingston HyperX KHX1866C10D3/4G
Serial number 7922C0D7h (3619693177)
Issue date Week 51 / 2015
Module size 4 GB (1 rank, 8 banks)
Module type Unbuffered DIMM
Memory type DDR3 SDRAM
Memory speed DDR3-1866 (933 MHz)
Module width 64 bit
Module voltage 1.5 V
Error detection method No
Refresh rate Normal (7.8 us)

ChannelB-DIMM0
Module name Kingston 9905403-478.A00LF
Serial number BC1D4E64h (1682841020)
Release date Week 46 / 2012
Module size 4 GB (2 ranks, 8 banks)
Module type Unbuffered DIMM
Memory type DDR3 SDRAM
Memory speed DDR3-1333 (667 MHz) )
Module width 64 bit
Module voltage 1.5 V
Error detection method No
Refresh rate Normal (7.8 us)

ChannelB-DIMM1
Module name Kingston HyperX KHX1866C10D3/4G
Serial number 7922C0D7h (3619693177)
Issue date Week 51 / 2015
Module size 4 GB (1 rank, 8 banks)
Module type Unbuffered DIMM
Memory type DDR3 SDRAM
Memory speed DDR3-1866 (933 MHz)
Module width 64 bit
Module voltage 1.5 V
Error detection method No
Refresh rate Normal (7.8 us)

Mother: Asus P8Z77-V LX
Proc: Intel Core i7 3770