My PC won't boot up correctly and has just lost half its memory!!!

As above, is this info enough?
64 bit Win 7 Professional
six core i7 970 processor
ASUS® P6X58D-E motherboard
Samsung triple channel DDR3 4GB RAM 1333 times six slots so 24GB.

Thanks for any advice .

On a system like yours where you have 24GB installed and only 16GB working, the reason is usually due to a memory timing problem. Here's what I think is happening to you.

Below is the Qualified Memory Vendor list for your motherboard

https://www.asus.com/Motherboards/P6X58DE/HelpDesk_QVL/

Any memory brand/model not listed is untested by Asus and is therefore officially unsupported. Any memory brand/model not listed by them may simply mean they did not test it, or it may mean they tried it and it did not run properly on the board. So you can't really know if your samsung memory was tested on your board or not. Turns out there is only one specific model of memory that will definitely run as 24GB on your board and its a Kingston KHX1600C9D3K3/12GX.

Running memory in dual or triple channel is a challenge for a motherboard because the timing has to be so precise across all dimms, as the bits from each dimm stream at full speed into the motherboard's memory controller which then joins the streams together like a weaving machine joins threads into a fabric, done at the full clock rate, quite a feat when you think about it, and that's why its important to use only the Asus-approved memory. If you download and run cpu-z, then select its SPD tab, you will see the exact part number of your current memory, slot by slot. Forgot to mention, you might just have a bad dimm, lol.

Assuming you want to keep your current memory, I see two options. One is to use trial and error by switching around your current dimms to different physical slots, marking them first with a pen or tape so you can keep track of which goes where, and hope you can get them installed into such a configuration that 24GB becomes active. Though they appear identical, each dimm is of course just slightly different from the others, and finding the right pairing may result in compatibility if you are lucky. The second is to try using "looser" i.e. more "relaxed" memory timing OR a slight memory overvolting, both of which you can change in bios. Either looser timing or more voltage can make your timing better synchronized. Before making either change, timing or voltage, BE SURE you know how to reset your cmos/bios manually, as there is a chance the new settings will cause your machine not to boot, in which case you'll need to reset to the default bios settings to get things back to normal. Changing bios settings like this is somewhat of an advanced tweak for the average user, so please consult your motherboard manual as well as read up on the topic, such as at the link below

How to manually set up your RAM speed, timings and voltage | Memory | OC3D News

EDIT: before trying any of the above, update your bios to the latest version. Worth a try.
Also - if this problem just came out of nowhere, its because though you were running fine before you were also just barely above the threshold of timing compatibility, then fell barely below it. This slight change can happen as the capacitors in your motherboard and power supply age, or slight resistances enter your system over time due to oxidation, or may other causes not worth going into. Just wanted to point out that its not that unusual for this to happen out of nowhere