Software companies want to keep you as a customer. If you can prove you purchased their product (via credit card statement, old email, or approximate purchase date), most will happily resend your license key.
The rise of subscription software (SaaS) is gradually eliminating license key problems. With subscriptions:
What (Windows, Mac, iOS, Android) are you using?
Most sites claiming to offer free keys or "keygen" executables are fronts for malware. Downloading these files can lead to ransomware, spyware, or keyloggers being installed on your system.
Before we dive into solutions, we must address the elephant in the room. If you type into Google, the first ten pages will be littered with links to shady websites offering "cracks," "keygens," and "portable finders."