If your network card is PCI and you're trying to plug it into the bottom slot, then you can't turn it around and expect it to work. PCI slots come in two different voltages - 3.3V and 5V, and the position of the divider in the slot determines which type it is. If the divider is on the right end (towards the middle of the board) then it is a 5V slot, whereas if the divider is on the left end (towards the board edge) then it is a 3.3V slot.
If your PCI card has only one notch then you have to match it to the correct slot in order to install it. Many PCI cards are dual voltage, with notches at both ends of the connector, and will work in either type of slot, but some cards only operate at one voltage.