Caller Line Identification can tell you who has used your broadband connection. Secondly you must talk to customer care representative of your broadband supplier to check the usage and account details of your broadband connection.
How Can I Tell If Someone Is Using My Broadband Connection?
I had this happen to me and my provider said what my machine was experiencing was called "piggybacking" where a company changed a registry and basically took over my computer when I wasn't using it. This would usually happen at night. The company was using my broadband connection. The fix was that I set a password before my computer could be booted up.