The application may already be running, check that it is not already on your notification taskbar listed in the grid of icons on the lower right side. Double click the ORIGIN PC logo icon. If not, there may be an issue with Sentinel and will probably need to be reset. Locate the installation folder for Sentinel, it should be under Program Files and in the Sentinel folder. Locate the file named Recover_Sentinel, run the file and then restart the computer.
The issue should be resolved now, if not, check this solution below for more a in-depth resolution.