I had the same thing after upgrading to 4.2.1
The iPAD refused to see the iTune - or maybe the other way around.
After a all night of "googling" the internet, I gave up.
Took my iPAD to work.
Connected it to my LAP top there (haven't got it at home that time) .
It immediately responded and started working iPAD-iTune "hagging" each other.
Bottom line. Something went wrong on my iTune Application at home.
So , my 1st advice is try connecting to another PC.
2nd advice if you don't have access to another PC, try uninstall and re-Install iTune.