If you open a link to a conversation and it doesn't load, you'll see one of two messages. Each one means something different.
"Conversation not found"
The conversation either:
was deleted by its owner, or
was moved to a project you no longer have access to, or
was never visible to your account in the first place.
"Access denied"
The conversation exists, but the link is to a workspace conversation and you're signed in with an account that isn't a member of that workspace. This usually happens when:
A teammate sent you a workspace link, but they meant to share with a guest. Ask them to share with you using Specific people instead, which works for guests as well.
You're signed in with the wrong email. Sign out, sign back in with the email that was shared with, and try the link again.