ReHIPS Agent is responsible for desktops management. It's the process that shows desktops widget and allows you to switch between isolated desktops. The widget tries his best to be on top of other windows, but different things happen. So even if some evil window is blocking the widget and you seem to stuck on the isolated desktop forever, don't worry, we have plan B: shortcuts to the rescue! Ctrl+Alt+i will fast switch you back to the main desktop (where for example you can terminate that nasty process from ReHIPS GUI) and Ctrl+Alt+F4 will just close the foreground window terminating the process.
And for the sake of completeness you can use F1 to show Agent first start screen with hint reminding about shortcuts. It's shown the first time you start an isolated program on a separate desktop, so maybe you remember it.