Macbook pro doesn't want to start, "no RAM installed" beeps (1 every 5 seconds) I have a problem with my Macbook Pro.

Everything was working fine up until yesterday; I can't start it anymore and it  "beeps" once every 5 seconds.

According to the support, this indicates that "no RAM is installed", which is not true.

So I tried to change RAM but I have the same problem. I put the suspect RAM into another computer, and it worked fine.

I Googled but I didn't find a solutions. The Macbook didn't take any knocks, I didn't try to change any RAM (before this problem), there is no reason for this error.

Does anyone have an idea ?

(Macbook Pro 13-inches mid-2012)
Sounds like your Logic (mother) board is done. 

You should see your RAM in the About this Mac.

Try resenting the SMC and the EFI.