Why does Windows 10 see all the memory but only work with a part of it?

Question

Windows

The computer has 2x2 + 1x4 GB, and until recently there were no problems.
Today suddenly the system stopped working with 6 gigabytes of memory, although it sees them (photo under the spoiler)
Why is this, and how to fix it?

spoiler

Answer 1

See Memory Remapping in BIOS, or something similar.
You can also swap modules, reset BIOS to default