Your email address will show as invalid if your Pro was bought from anywhere apart from the official site or if if your Pro benefits are out of date. If you didn't ever recieve an email from LW, how did you download Pro to begin with? Have you been able to update Pro since you bought it? If so, how did you update? When did you buy Pro & who did you pay for Pro?
When you buy Pro, you recieve an email from LW containing a link to your Pro personal download page & that's where you download from.

It's up to you whether you buy Pro again or stick to the basic (free) version. Did you find the location of your saved folder? If your saved folder's in the default location, or another safe location, you can reinstall LW without losing any of your saved files. What version of Pro were you using before you were prompted to update?