eth0 "disabled" after upgrade Squeeze to Wheezy

[Lisi Reisz <lisi.reisz@gmail.com>]

I have just upgraded a client's computer to Wheezy.  It appeared to go 
well and there was certainly an internet connection: it would not 
have been able to upgrade otherwise!

Now there is  none.  I have checked /etc/network/interfaces and 
changed "allow-hotplug" to "auto", just for something to try. :-(  It 
made no difference.  I pinged the gateway, largely so that I could 
report that I had done so.  I got the error message "Network is 
unreachable". 

KControl tells me that eth0 is "disabled".  How and why is it 
disabled?  More importantly, **how do I enable it?**

Thanks,
Lisi