Based on your connection string you're attaching the database. Chances are that the database which you're attaching is included in your project and being copied from your project output directory every time you start the application. This means that every change you have done when the program was running gets overwritten by the initial copy when the program starts next time.
Try removing the database from your project and moving it to a static location from where you attach the database.
Another observation, at the moment you concatenate values directly to the SQL statement. This leaves you wide open to SQL injection and other problems. Instead you should use bind variables. Have a look at Properly executing database operations
Third observation (or actually a guess). Are you perhaps storing user name and password. Storing password as plain text should never be done. Have a look at Password Storage: How to do it.