I can click on the curve, but no curve editor comes up.
IF I click on it, the line turns red, but nothing else.
HDRP 10, Unity 2020.2.6f1.
The curve editor for particle curves is built in to the particle Inspector - it should be editable at the bottom of the inspector.
I dont see it in the inspector, but I did find it in the particle effect editor, in a panel to the right.
In the inspector window, there might be a bar at the bottom that looks this:
If you click on the ==== it should expand to show you the curve.
If that’s missing, the vertical 3 dots on the right might have been clicked on accident which has to option to break out that as a separate “preview” window. It’s possible that preview window got dragged off of the screen somewhere. When editing a particle system, the UI elements to redock it to the inspector aren’t visible. You’ll need to select an asset in the project view and click on the vertical 3 dots in the bottom right to snap it back to the inspector.
Ahh, they hid it away in the preview window which I always have minimized!