1) It might be an issue with the spam filter (this does happen at universities!). Consider writing from another address, or ping the secretary/ whoever your contact person is in said dept asking them to reply to you... if they don't it's the filter.
2) Just write again with your original email below your new one, saying something on the lines of "hi I wrote you last week but did not receive a response, and was wondering whether you've given this any further consideration." Profs are busy people and often lose track of queries like that in my experience, so I'm sure a few of them are just in that category (assuming #1 isn't true!).
I mean if you're already accepted they want you to be there, so I wouldn't assume they hate you or anything like that.