This is a problem I’ve only ever had on Windows 7 but I gather it can affect you on XP too.
If Evernote says
Synchronization failed, could not connect to Evernote server
and you aren’t sure why because everything else seems to work on the internet, go to Tools menu, Options, then the General tab and then click on “Log folder”.
If in recent logs you can see
Windows Internet subsystem is in offline mode
chances are that Internet Explorer is in offline mode. Fire up Internet Explorer, verify that it is browsing the web again, and you should find that Evernote is working again too and will connect to the servers and update.