If your 6-month-free-upgrade period hasn't expired yet, go
here and enter the email adress you used to buy it in the first place.
If your 6-month-free-upgrade period
has expired then you have to pay for Pro again. If you'd bought the CD version, poping the CD in the CD drive of your comp and installing it from there is doable, but you wouldn't be able to update your Pro version. You'd have to downgrade to Basic, and either update the Basics or buy Pro again.