Installed RAM 12gb only 3.95gb usable?

Solution
Remove the 4 GB stick and leave only 8 GB stick. Try to boot. If all works fine and Windows recognizes correctly 8 GB, that would mean your motherboard has problem running both sticks together now.
If you can't boot with that stick alone, that would mean either it had failed, or the slot it is in has failed. Move the 8 GB stick to other slot and repeat the boot. If still no boot, it is faulty stick, but if it works fine at other slot, it would mean the other slot has failed.
May be it is a over the limit of memory of ram your motherboard can support
Maybe your motherboard can support upto 8gb only and maybe your one stick 4 gb and other 8gb your mobo cannot support that 8 gb ram. Maybe that is the reason it shows 3.95gb
 


What motherboard do you have? Why 12GB and not 16? Are you on x58? Windows operating system 32bit? 64bit?
 


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I have 12 because that's all that I could afford at the time.

I don't know what x58 is

64bit operating system

It all worked a week ago for about a year

 
Remove the 4 GB stick and leave only 8 GB stick. Try to boot. If all works fine and Windows recognizes correctly 8 GB, that would mean your motherboard has problem running both sticks together now.
If you can't boot with that stick alone, that would mean either it had failed, or the slot it is in has failed. Move the 8 GB stick to other slot and repeat the boot. If still no boot, it is faulty stick, but if it works fine at other slot, it would mean the other slot has failed.
 
Solution



Even though Windows and my bios both see that the RAM is there?

 
What version of Windows do you have? 32 bit versions only support 4GB of memory and it comes out to about 3.95GB. If you don't have 64 bit windows, that is your problem. I see you say you have 64 bit, but you might want to double check that because it is the most probable reason that only that exact amount of memory would be recognized.

The other, second most probable reason would be that you have incompatible memory modules. Different memory modules, and even memory modules that are the exact same part number do not always play nice together which is why we ALWAYS recommend purchasing memory in matched sets that have been tested and verified to play nice together, both at ALL and in dual channel operation for systems that support it.

It could also be that you have different modules which do not have the same voltage requirements and what is working for one module may not be sufficient for another one. Or, you could simply have purchased memory that is failing. Just because it shows up in the BIOS does not mean that it passes POST testing or that it will work once the operating system takes over and begins running it's control systems.
 



I had it working not even a week ago though..

 
Do you mean to say that one of the memory modules, or more than one, does not function in one of the memory slots?

Is this when it is the only memory installed or is memory not working in that slot even when other memory is installed in other slots? Does it show the same results regardless of WHICH memory module is installed in that slot?

WHICH slot is it? Some memory slots will not work on some chipsets if there is only one memory module installed and it is not installed in the slot designated for installation with only one memory module. I wouldn't necessarily convict the motherboard yet until we determine that all other possibilities are exhausted.
 

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