There is no context in the question.
In general with ISDN, you should first have
a look at "sh isdn stat"; if layer1
is activated, you are good; if it's
Deactivated, there is a physical layer
problem (cisco port, cable, or carrier)
that can best be addressed by confirming
the circuit and changing ports and
cables.
Note that outside of North America, it's
often the case that layer1 is down in
between calls (ie the TEI is negotiated for
each call with the "isdn tei first"
call command).