How to address bad research experience
If I had no further information about what happened and it was me, I'd not mention it on application materials. That is, not try to explain it away. List it as a research experience on a CV, without other explanation. I'm assuming the grade you got is on a transcript. So, it isn't something you are hiding, but just not emphasizing. 

If you are asked about it, you can honestly say that you don't understand the grade and that your supervisor stopped replying to your updates. You don't know why that was. Things happen. They aren't your fault. If the experience itself was valuable in setting your thinking then it was a net plus in spite of the mediocre grade. You can say those kinds of things. 

But, as I mentioned in a comment, it would be good to also know why it occurred. I think I'd refrain from explaining it in written materials in any case, but you would have some additional background if the issue comes up.