Windows 10 and BIOS does not detect one RAM slot anymore

Hi there.
I've been working with my laptop for a couple of years.
It was manufactured with 6GB of RAM, one 4GB placed in slot #1 and another 2GB placed in slot #2.
I was actually working fine with it, but suddenly i figured a big performance problem in my laptop, after two days, i surprisingly found out that windows 10 detects only 2GB of my RAM (which was actually detecting all 6GB in the past days!)
so i ran a few tests, cpu-z fully recognizes my RAM, as you can see in the attached screenshots, and also RAMMon can detect two RAM slots with total amount of 6GB capacity. But it says "Total physical memory :1931MB (6144MB)" which i don't know what it really means.
In windows properties i see about 2GB of ram, and also BIOS only detects 2GB!
So what's going on?
Does anyone have any idea what just happened to my system and how am i gonna solve it?

Do you have a large amount of RAM reserved for hardware? Have a look in the memory section for the Performance Tab in Task Manager. See if there is a large hardware reserved amount.

@philc43
no there is just 116MB free!
An also task manager says 1 of my 4 slots is used!

philc43 said:

Do you have a large amount of RAM reserved for hardware? Have a look in the memory section for the Performance Tab in Task Manager. See if there is a large hardware reserved amount.

i just checked it and there is 4GB of hardware reserved Memory.
And task manager detects 2 slots of Ram (it was 1 slot before)