Tough break Jim. Rest up!
If this is the first time you got hit hard with the dizziness, take a look at your front squat rack position. Its pretty easy to get the bar in a position where it pinches the carotid artery. You won't feel it, but the experience is usually spot on with what happened to you today.
On another note, a slightly bent bar can hit the artery even if the rack is good. I had this problem. Ended up having to mark my bar with a dot, and roll it when the bar was in the hooks so that when I unrack it, the curve of the bend faces away from my neck.
May or may not be the case but...