RAM cannot be detect

Reset  SMC:   How to reset the SMC of your Mac - Apple Support   Choose the appropriate method.

    Quote:

    Resetting the system management controller (SMC) can resolve certain issues related to power, battery, fans,

    and other features.

Many times the slot nearest the Bottom Case fails due to cracked solder joints on the memory slot, but I have also seen a few failures of the other slot nearest the Keyboard. If both RAM modules work in the one good slot, then most likely the memory slot is bad.

Also the memory in the slot nearest the Keyboard can pop loose when inserting the second module nearest the Bottom Case since they share a locking arm. If the issue is with the RAM in the slot nearest the Keyboard, then make sure to press on the outside edge of the RAM after inserting the second stick to make sure the first module is still latched into place.

Have you been using this memory for years or are you performing a memory upgrade? Are you using the original Apple memory or a third party upgrade? Macs can be very picky about the memory used so it is best to only use memory from Crucial or OWC (even then use only the exact part numbers suggested by using the tools on their respective websites).