If you have a previous account on wikitree. You can start with that account
If you lost your password Please see: Lost password
If you don't receive the e-mail Please see: E-Mail Confirmation Problems.
Once your online you can merge the 2 accounts Please see: Merging
If it's something other than these 3 issues please send an email to info @wikitree.com explaining the situation with all relevant email addresses used and they will help you sort it.