These are the possible reasons:
You requested the password reset in browser X, but when you clicked the link in your email, it opens in browser Y.
Solution: You must use the SAME browser to reset your password.
The link in your email will expire after a certain period of time. If you clicked this link AFTER this period of time has expired, it won't work.
Solution: You need to click the "reset your password" link again, and make sure after you've done that, you immediately check your email inbox and click the link in your email immediately.