First, try the following:
Find the SyncBack icon in the system tray (the icons that are listed next to the time, usually in the lower-right of the screen) and double-click it, or right-click on it and select Show from the pop-up menu.
If the window does not appear, but SyncBack is listed in the task bar at the bottom of the screen, the window may be appearing off-screen. This can happen, for example, if you previously had two monitors and the monitor that SyncBack was displayed on is no longer connected. To fix this go to the command prompt, go to the folder SyncBack is installed into, and enter:
If you are using SyncBackPro: syncbackpro.exe -posreset
If you are using SyncBackSE: syncbackse.exe -posreset
If you are using SyncBackFree: syncbackfree.exe -posreset
Then exit SyncBack and run it as per normal.
If the problem is still not corrected then export your profiles, uninstall SyncBack (and choose to have the settings deleted), reinstall SyncBack, and import your profiles.
If that still does not correct the problem check to make sure your display settings are correct in Windows. SyncBack will display windows on your main/default monitor.