According to 2 Thessalonians 2:1-3 some of the Thessalonians seemed to think the day of the Lord had already come, and they were frightened by the thought. Paul writes 2 Thessalonians to tell them that it has not come and the other things must happen before the second coming (2:1-12).
Paul addresses another problem that was occurring in the church which was some Christians were being lazy and not working because they could just get money from wealthier Christians (3:6-15).
Throughout this letter Paul encourages the Thessalonians in the face of persecution they have been facing (1:4).