ssh tunnel suddenly stops working

The cause of this problem was hard to track down... :-(

I sometimes open a tunnel to another machine, so it can serve as a proxy for my local box:
ssh -D9101 other.pc.net
(the port number can be any number > 1023)

Suddenly this started failing with the following error:
bind: Cannot assign requested address
channel_setup_fwd_listener: cannot listen to port: 9101
Could not request local forwarding.

It turned out that one thing that had changed, i.e. switching the PC from a wired to a wireless connection, was causing it: the loopback device was no longer up...

To check if it is there: ifconfig should show:

lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 (and some more text)

To bring it up, run ifconfig lo up (as root)