I'm starting with the assumption that you have tested the card and the cameras and they are ok. In that case you need to check 3 things:
1 - make sure that the cables connecting the board to the cameras are intact. If they are then go to step 2
2 - make sure you are using the latest version of the software - 8.12 - and it's correctly configured (the image settings are set accordingly) : 60 Hz NTSC if you're in US, 50 Hz Pal if you're in the rest of the world and a lower resolution - no more than half D1, your card is very weak and will barely support even that. If all is set as it should then
3 - there's a software conflict and the fastest way to fix that is to reinstall the system from scratch.
If you don't know the software version or you need to update it then go here
http://www.geovision.com.tw/english/5_0.asp