As a person who has been suicidal for years(currently diagnosed with severe depression), I feel like lots of people know that something is wrong and they just choose not to do anything about it because they don't want to overstep their boundaries. For instance, my mom asked me about some razorblades in my room that she had found, I said it was nothing and she left it alone. I always wished that someone would pressure me about it rather than shying off when I pushed them away.
Lately I feel like I've undersold my depression. I never wanted to burden anyone and everyone just abandoned me because of it. Maybe if they thought I wasn't just feeling sorry for myself they'd have done something about it.
Long story short, don't give a fuck about what they say. If you feel something is wrong with someone, ask them about it. Really, FUCKING ask them about it. Don't take no for an answer.
THIS. This, this, this. I also have suffered with severe depression, and every time someone asked me if I was okay or pointed out that I wasn't myself lately, I'd lie and push them away and say anything to stop them asking anymore questions. If you think someone is unwell, ask and ask and ask. Make sure they know you're there for them and that you'll always listen and never judge them.
Also, it's VERY important you don't blame yourself for what happened. When I was suicidal, one thing that really worried me was exactly that - people feeling awfully guilty for not having realised I was unwell and for not having offered a helping hand. Honestly, you shouldn't let those thoughts or ideas get to you. I guess it's natural to have those thoughts, but it's important to know that it's not your fault.
Sending huge hugs your way.