I had a weird problem, and I want to record the solution here so I can find it next time!
Whenever I started RegExPal, the window would open in the “background”, but I couldn’t figure out any way to bring it to the foreground. It appeared to be an empty window, like when I hovered over it. A restart and re-installation of Paratext 8 didn’t help. I found on this forum how to start RegExPal without Paratext (“Paratext.exe -regexpal”), so I tried that, and again the icon appeared in the task bar, but the window did not appear on the screen. But clicking on that icon multiple times showed that the window was clearly located off-screen.
No problem, I thought, I’ll just use Windows key + arrows to bring it back onto the screen. But that didn’t work. Eventually the only thing that worked was SHIFT right-clicking on the RegExPal icon (when it was run separately from Paratext) and selecting Maximize. That brought it back to the main screen, and then I could drag the title bar to get it out of maximized mode. (If I just minimized again, it would again go off-screen.)
So the reason I had this problem is that I have an external monitor in my office, and had put the RegExPal window on that external monitor for work I was doing, but when working at home I didn’t have the external monitor.
I didn’t try on Paratext 9, but maybe it would be a nice feature improvement would be to check for the existence of the external monitor when you open up RegExPal before putting the window out there! Also, if there is some way that it could respond to Windows key + arrows, that would be helpful. (Although when RegExPal is on my main screen it seems to respond to those keys just fine.)