A complete sentence has both a subject and a conjugated verb. A fragment is missing either the subject or the verb.
Beginning a sentence with a subordinating conjunction may create a fragment
Right: Adam ate the apple.
Wrong: Because Adam ate the apple.
Substituting a semicolon for a comma may also create a fragment.
Wrong: Adam worked in the garden; content with his role.
Right: Adam worked in the garden, content with his role.