FATAL: No bootable medium found! System halted

In virtualbox machines there are two options in order to configure the virtual disks: one is fixed size and the other one is variable size.

If you choose fixed size in one virtual disk and after that you decide to overwrite your existent disk with the option variable size then the system will only boot once because once you install any application or write some data to your virtual disk, the disk size will change (remember the the virtual disk runs from memory--→RAM) but once the changes has to be written to the disk (reboots or shutdown the machine) the system will missconfigure.

I’m not an IT Programmer or anything like that so the only way I know how  to fix this is to create a new machine and never use fixed size and always use the option read/write option