msquared-> I assume you are using a LM2000 card:
This mostly has the following reason:
Corruption of the cfg file, or the NIC has become unseated or failed
To solve it do the following:
Try reseating card and checking the dipswitch settings on the LM2000 card. Default: IRQ 5, I/O 200 is switch 1-3 on, 4-5 off, 7-8 on. Default values must be set for DOS Sniffer to work. If this fails, delete the SIA software and reinstall.
If still fails you might have a faulty NIC card.
rasdebol->What system are you using? what NIC etc.?