Try and read the Transaction Logs to see what happened.
Unless someone knows of an easy way I reckon that is "very hard" to do, in practice
Start trace in SQL Server profiler and checked events(TSQL-SQL:BatchCompleted,SQL:BatchStarting,SQL:StmtCompleted and SQL:StmtStarting)(Recommended).
That requires that Profiler was running at the time. Maybe it was, but if it was then I reckon the O/P would have known about it, and how to use that to solve the problem 🙂
FWIW we do not run Profiler continuously in order to be able to recover from a situation like this, we use Triggers/etc. to save Audit data to enable us to figure out, and reinstate, any accidental/fraudulent data changes