This error indicates that the network link is down. The network driver does not provide any further details.

It is normal for some network cards to take a few seconds to establish the link.  You may therefore see messages such as:

    net0: 52:54:00:12:34:56 using rtl8139 on PCI00:03.0 (Ethernet) [open]
      [Link:down, TX:0 TXE:0 RX:0 RXE:0]
      [Link status: Down (http://ipxe.org/38086101)]
    Waiting for link-up on net0... ok

These messages are not a cause for concern; this is expected behaviour for these network cards.

However, if you see a message such as

    Waiting for link-up on net0... failed: Down (http://ipxe.org/38086101)

then you should check that the cable is plugged in, and that any link lights are on.