You may have gotten something that looks like this. It may look different depending on your Windows OS version, but the message is very similar:
There are three reasons why you may be getting this message.
1. The PhotoSpring Windows app runs as a service. What this means is that when your PC boots up, PhotoSpring should load on its own and run in the background.
2. If you just installed PhotoSpring, it may already be running in the background
3. When PhotoSpring is open, it may be behind other windows.
So if any of these are true and you try to open PhotoSpring again you will get the "instance is already running" message.
Instead you can access PhotoSpring from the system tray in the lower right hand corner of your screen. First locate the carat which looks like this < or this ^. Click on the carat. It will open up background services that are running.
You will find the PhotoSpring logo inside. Just right click on the icon to show the options menu and then go to Settings within the menu to open the config screen.
Windows error, multiple instances running