Why did Stack Overflow delete my account without any warning? ... I had just upvoted one of my friend's post.
First, a moderator doesn't delete an account without any good reason to do so. Your own remark makes clear your account was deleted for being a serial voter or a sock puppet, someone who votes for a user instead of his/her posts. Since we find quality important, this is not allowed. It is cheating! I seriously doubt if you really just voted once, or if you voted a lot more. In fact, you know what is wrong since you say you just voted once. Maybe you have multiple accounts that voted multiple times on the same user.
The worst things is that they did not answer me and keep ignoring me.
Give it some time. Moderators are very busy moderating the site and handling cases like this. Usually it will take a few days, a week maybe, before they respond. (Since you are on the naughty bench, don't expect they run to you immediately if you 'scream' to them)