"Can't open FF because FF is already running"

That is the message I get today no matter how I try to open Filter Forge. The message of course uses the full name instead of FF - I just tried to make the headline a bit shorter
But how do I get out of that loop?
It's fine, that the program is already running. But I can't access it neither as a standalone program or from inside Photoshop, as I normally can. This started earlier today, when I, as I also normally can, was downloading some new filters from FF's own page. I have rebooted the computer, but that didn't help.
Any and all advice will be much appreciated

Thanks a billion for answering me.
Yes, I might have been too hasty. Impatient. But the program stayed unresponsive until I rebooted the whole computer, and wouldn't open for me after that, because it was already running, so I don't know.
Just in case I wasn't too eager, where do I find that Task Manager, please?

Thank you ever so much. I will try that, then.