Hi,
I am assuming it's a windows machine, if it's a mac this does not apply,
Have you got the firewire port in the computer enabled? or it has somehow become disabled?
Do the following with the MOTU plugged in:
To check go to Control Panel > network connections > (Right Click on) 1394 connection.
If the list that comes up says disable at the top that is correct (it is actually enabled and is asking if you want to disable), if it says enable click on it to enable the connection.
If it is enabled try clicking on "Repair" and see if this fixes the problem.
Cheers
Alan.