Can two people genuinely go back to being just friends after feelings get involved?

Two people have been friends for about 4 years. Somewhere along the way, feelings became involved, but one person wants to keep the relationship platonic. The friendship and history between them still matter to both, although there has been some hurt because of the situation.

For anyone who has been in a similar situation, what happened afterward? Were you eventually able to have a normal, comfortable friendship again?

I'm especially interested in hearing from people who were able to stay friends, as well as people who ended up losing the friendship. What do you think made the difference?