From @user3344003's answer to the question, Can/do representatives move between the senate and the house in US congress?
It is uncommon, but not unknown for Senators to become Representatives. Claude Pepper was a Senator from Florida who was defeated and returned to Congress as a Representative. John Quincy Adams served as Senator, then President, then Representative. Henry Clay was a Senator, the speaker of the House, then Senator again. (This is not a complete list. There are some others I know of as well.)
From @Micah in a deleted answer,
Andrew Johnson was President from 1865 to 1869, and then served briefly as a Senator in 1875 before his death.
The following list includes those who, at one point, were US Senators; but ended their political careers as Representatives.
John Adair KY
Thomas Benton MO
Henry Blair NH
Charles Buckalew PA
Berkeley Bunker NV
John Condit NJ
John Crittenden KY
Franklin Davenport NJ
William Eaton CT
Magnus Johnson MN
William Kellogg LA
Alton Lennon NC
Lucius Lyon MI
Jonathan Mason MA
John McRae MS
Hugh Mitchell WA
Samuel Mitchill NY
Cameron Morrison NC
Jeremiah Morrow OH
James Nesmith OR
Thomas Norwood GA
Claude Pepper FL
Timothy Pickering MA
Charles Pinckney SC
Luke Poland VT
John Pope KY
Luke Pryor AL
Robert Rantoul Jr. MA
Philip Reed MD
John Robsion KY
James Schureman NJ
Theodore Sedgwick MA
Richard Stockton NJ
Charles Towne NY
James Wadsworth Jr. NY
Albert White IN
Washington Whitthorne TN
Morton Wilkinson MN
Garrett Withers KY
Robert Wright MD