Computer authorization suddenly is failing

I have used my iPhone for several years now with my PC. I've never had a problem with authorization of my PC while I sync my phone.

Recently, I have to enter my ITunes password. I am confident I am entering the correct password, but the computer is "no longer authorized" and iTunes threatens to remove all the apps I've stored on my phone. Clearly I don't want this to happen.

What' gives? What's going on? Any solution ideas?