Usable ram and missing bios screen

Hello there , this is my first time using this forum to confront about my problems. Im currently using mobo Asrock b450 pro 4 , processor AMD ryzen 5 2600, 16GB Ram, system type is 64 bit Win 10, Graphic card gtx1050ti .

I currently have a problem with my ram it said it only use 8gb instead of 16, My bios detected all of my rams and my Windows also detect all of my rams but only let 8gb usable . It started when my maid accidentally nudge my pc. I have search around the internet and found various solution. At first i tried using msconfig , virtualization, paging file, and tweak a little bit in the BIOS. Some user told me to enable the 4g decoder feature in the BIOS, when i enabled it, My pc keeps skipping BIOS screen and now i can't fix it ( Note at that time i also enabled boot with UEFI or sth i don't clearly remember). I tried looking how to dealt with it. I know it is not because of fast startup since i haven't enabled it and i can't go to UEFI firmware since im using legacy and i can't resort to reset CMOS since my mobo is only 2 years old and im afraid that will damage my mobo. This problem actually already happened in the past but it was fixable by using the msconfig and plugging and unplugging my RAM. If you ask why i don't plug and unplug my RAM well because right now im in college years and i need my pc to attend college, if unplug and plug my RAM there is a chance where my pc won't turn on again. I hope you guys can give me some advice or step how to fix this problem since there are a lot of assignments i need to finish before deadline. Thank you for reading.

*sorry for my bad english*

Just to be sure , what will happen after i restart UEFI to designated version ? Does restarting UEFI will also restart my BIOS setting to default ?

I did what you asked me to do , but it seems my pc still skip BIOS Screen and if i spam f2 (which is my key for entering BIOS) my monitor will only show black screen only

SpecPep wrote: Update , i have fixed the bios screen by fixing the ram position but it still only use 8gb instead of 16gb and not only that is not even 8gb it is 7,9 gb. This irritates me, is there any way to fix this ?

my ram is corsair vengeance lpx(2x4) 8gb DDR4 kit 2666mhz.

Hello i done what you have told me and did some research. At first i try them one by one and my mobo still recognize them as single channel , so i look into my manual again and the ram position for dual channel is a2 b2 with the frequency 2400mhz. I did that and it work but when i inserted all of my ram while using the manual guidance ( which is 4 ram dual channel will be recognized if it runs on 1866mhz ) it doesnt work and if i put higher frequency than 2133 my pc wont turn on. Whose at fault here is it my ram or my motherboard ?

EDIT : So i tried your advice again and my 2 RAM didnt work like the other 2. The other 2 can be identified by my bios as dual channel and used 7,9gb. But beside those ram imy bios detect them as single channel and only used 4gb. What should i do now ??