[Client gui will not open on some]

Starting this AM some of our clients can't open up the gui.  Rt click on the eset icon and nothing happens.  Even tried opening up through programs.  Other clients are ok.  We are on latest v6.3.  Any ideas?

[Can't open client on desktop]

We are running v6.3 for a month and suddenly today multiple clients can't open up eset on their desktops.  Some can as usual and others can't.  Click or rt click to open up the client does nothing..  The client is updated and current.  You can mouse over the icon and it shows ver, but clicking does nothing.  Any ideas?