this is just a wild guess - but hold both the Shift and the Control keys while you start up RSLogix500 ...
if it shows up as running in Task Manager - but not on the screen - maybe it's just been moved too far left, right, up, or down to be seen ... Shift and Control while starting it up, should bring it up in a "default" position - and maybe you'll see it then ...
like I said, it's just a guess - but quick and easy to check ...