Your course selection and outside reading probably reflect your own interests. Look in your course notes, copies of books, blog posts, diary entries to see what your starred, put exclamation points by or thought "OMG, that is the coolest idea ever." Did you find historical assumptions you disagreed with, statistics that you couldn't believe were true, find people who did amazing things, become intrigued by what people ate, or what other people put in a museum. Follow these interests as they may lead to a topic.
Your course work was likely divided into areas (World, Europe, North America), fields (modern, political, social, cultural), and themes (for example, gender, migration, revolutions). You have probably also narrowed your course selection by time period. While it is unlikely that you will be required to choose a topic that addresses each of these, selecting a topic which includes at least two might be advisable, particularly if you are applying to a PhD program. For example, you might examine "Being a Man in London: Changing Ideas about Masculinity During the Reign of George IV."
These individuals are important; do not ignore them. It would be unwise to try and write a thesis in an area of history if your department doesn't have any specialists in that area. Be aware of what topics, themes and time periods advisers are willing to supervise. Additionally, advisers ought to be a repository of thesis topics or leads. Tell them what interests you and they might be able to point you in the direction of relevant primary documents and secondary sources.
If you are really stuck, consider a monograph. Focus on one item -- one building, one person, one set of documents, one event, one town. Then use that one item as the case study and place it in its larger historic context. For example, if a store on the corner in your town has historical significance, you could research "The Importance of Smith's General Store to the Economic Development of Anytown during the Market Revolution, 1815-1830."
Check local repositories for primary documents. Your thesis must include primary research and there is usually not much money available to support research for thesis research, thus it would be useful if the primary sources were available locally or were on microfilm for use in the university library. Diaries are particularly useful, depending on the contents, as they often be used to investigate topics ranging from local attitudes towards national events to family dynamics. Newspapers can be used to analyze regional responses to national events. For example, if you had access to Cleveland newspapers from 1960-1965, you might write about attitudes toward the assassination of President Kennedy as represented in the Cleveland Press and the Cleveland Plain Dealer between 1963 and 1964.
Trends exist in historical research as they do in fashion and music. While in the 1980s topics involving the counties of the Communist bloc were popular, today scholars are leaning toward investigations of globalization or the Islamic world. If you are really stuck for ideas, it often doesn't hurt to be trendy.