I'm not sure, but, I think in the BIOS of this PC you can check if the PCI slot in which the 16P is installed is turned off. If it is, you can try turning it back on.
However, it may turn off again automatically if the 16P is drawing more power than the slot can provide.
The 16P was designed to use the 5 volts from the power pins of the PCI slot. The signaling pin (aka digital I/O pin) can be either 5 volts or 3.3 volts, but the power itself must be drawn from the 5 volt pins. Traditionally, there are 5 volt power pins and 3.3 volt power pins in a standard PCI slot.
I suspect that this riser card assembly is plugged into a PCIe type slot on the main board or possibly into a PCI slot which is specifically limited to 3.3 volt cards. In either case, it is unlikley that the riser card assembly has the required 5 volts on the power pins required to operate the 16P. If this part is true, then you will not be able to use the 16P in this PC.