By default, Hard Disk Sentinel runs as application. You can switch to service mode on the Configuration -> Integration page, but until that, it always works in application mode. Did you previously switched to service mode there?
If you switced to service and "manually stopped" it and launched HDSentinel.exe - then it will load as application again, NOT run as service.
You can very easily check in Windows Task Manager: please check the user name next to HDSentinel.exe
If you see "SYSTEM" - then the software runs in the context of the SYSTEM user ( = works as service) but if you see your user name, then it works as normal application, not a service.
If you do not see its main window when you simply launch HDSentinel.exe , then it is possible that the window positioned outside the screen (can happen if your resolution may changed and/or if you use multiple monitors). I'd recommend to try to completely close HDSentinel.exe from the Task Manager (to prevent running) and delete the file HDSentinel.ini from its folder to restore factory default settings and clear saved window size/position. This does not clear any statistics, logs, license - just the settings.
Then, launching HDSentinel.exe will display its main window, allowing access to all functions.
Side note: if it works as service, then there is no need to "manually stop" (I assume this means killing from Task Manager or stopping the service at Control Panel). There is a "Stop Service" shortcut in Hard Disk Sentinel program group which stops it - and also ask if you want to switch back to application mode by uninstalling its service and launching HDSentinel.exe (as application) exactly to allow access to all functions/options.