Can I recall a declined lead?

If you're talking about "opportunities", just go back since Thumbtack doesn't respect your distance or service settings it will return to "all services", 100 miles, it will re-appear.
In fact, thumbtack will show me "opportunities" I've already responded to when I click "back".

For "leads" check your email. You can follow the link back to that inquiry. Additionally, it may appear in messages since Thumbtack will show you all the Users who messaged you without much fuss. To find Users you messaged via "opportunities" type an initial and search. If you do not recall the User's initials, there's only 26 letters in the alphabet.

Good luck.