Ryan is right, we need more information to help. But I made this list of common collision issues so maybe you can take a look and see if your issue is on this list.
Collision Not Working Checklist
There’s a lot of ways that collision might not work as expected. Here are some reasons I’ve found. They are somewhat sorted in order of how likely I think they are to occur. None of these reasons are specific to 2D or 3D.
General Checklist
You are running the wrong scene.
The Run Project (F5, the play button on the left of the other buttons) button only runs the main scene, which might not be the same thing as the scene you currently have open.
Run Current Scen…