Arduino mini losing memory

Hi

I'v a problem. I've 2 arduinos mini, they work well just after uploading the blink sketch.

If i switch off them, then switch on, nothing appear.... they seem to hibernate.

I try to burn (with success) the original bootloader, the optiboot bootloader, they have the same behavior.

If someone know what happen, this would help me, because i'm blocked in my project.

Thanks

Gilles