Ram Upgrade for Optiplex 7040

@tibi500cid  is correct if for the 7040M but you did not state which form factor you have. For the others (MT, SFF) it is 64 GB as shown here. Assuming you are using DDR4 2133 then it could be a matter of RAM incompatibility. Did you purchase from Crucial as other makes of RAM can cause problems in a Dell system as Dells are very touchy about RAM makes and types. See the Crucial types here. And though the Crucial specs out as higher than 2133 it will run at 2133.

According to Dell spec:

Memory 2,3 Up to 4 DIMM slots (2 for Micro); Non-ECC dual-channel 2133MHz DDR4 SDRAM, up to 32GB (16GB for Micro)

https://clascsg.uconn.edu/download/specs/O7040.pdf

You have OptiPlex 7040 SFF with 180W up to 85% efficient Power Supply (80Plus Bronze). Use Dell support website and service tag then look inside system spec.

MT, SFF, micro (left to right)

PS community moderator may redact your posted service tag.

how do i locate the moel. 

After aditional help in the community I have the 7040 SFF. It performs fine with 32Ram installed but will not bot up when I install the remaining two sticks. Each stick is 16GB DDR-IV 2400MHZ from Arch Memeory

Arch Memory is a retailer that sells a number of different RAM types from various manufacturers. Not knowing what make of RAM your original 32GB is and what make the additional 32GB is, it is only a guess that it is RAM incompatibility. Again Crucial RAM is the most compatible RAM for Dell and if you are mixing 2 different makes of RAM that can cause the problem you are having.

Just a data point, not a solution, but I currently have a 7040 SFF that will not accept ANY RAM in slots 2 & 4. You can take the same RAM from slots 1 or 3 and put in 1 & 2 to enable dual channel and you get a POST power button orange LED flashing error code advising a problem with the RAM. This is even with the recommended Crucial RAM & Dell supplied SK Hynix. I can put 4GB, 8GB, or 16GB in slots 1 & 3 of Crucial or Dell supplied SK Hynix RAM with no problems, but use the same RAM in any configuration using slots 2 or 4 (1+2, 3+4, 1+2+3+4, etc) and no boot. I'm thinking it's just a faulty mainboard maybe? Weird fault though. Could also be Crucial & SK Hynix don't play well together because I don't have 4x identical pieces of known working RAM.