It depends ... it is pretty easy to hack a common person's e-mail. Look them up on FaceBook (if they have an account there).
Did they leave their e-mail address publicly available? Now you have their e-mail address, all you need is a password.
Look over their profile, noting the names of pets, significant others, family members as well as any publicly mentioned interests, celebrities, whatever.
Use variations of those names of pets, family members, etc as a password, if the account the e-mail is on requires numbers, toss 123 or the age of family member at the end. If the person was a fan of a particular car, try the model and year, etc. You get the idea.
The above will fail more often than not, but sometimes you* get lucky.
*I say you, but of course neither you nor I would be so malicious as to go breaking into an e-mail account.