I just got the monitor today and it worked normally. I inserted the network card into the pci slot or whatever and then when i turned it back on the monitor came up with that error.

I checked the cables and unplugged/plugged them back in, it's secure and everything, but it still won't work. Should I test another monitor on my computer to see if it is the monitor that is broken...or could it be something else?

you may have dislodged the graphics card when you fitted the network card. Check it.
Make sure it's pushed in properly at both ends of the card and not tilted.

I've fixed it up, I just took out the network card, I didn't need it anyway...because I'm just going to buy a wireless one, so hopefully it works when I put the wireless network card in.