If you dispute it, it is up to the claimant to release the claim or reject your dispute. If they reject the dispute, the claim will stay on there. Nothing negative happens to your account as a whole. It only affects that video and the way it affects it depends on what the claimant decides to do, such as put ads on it.
If they reject it and you still wish to fight it, you can appeal the rejection. If they still don't want to release the claim, they will have to have your video removed and THAT will give you a copyright strike. Then of course you can submit a counter notification and they will have 10 business days in order to prove they are going to sue you in order to keep the video down. Otherwise it will go back up and the strike will be removed. If they don't even own the song on your video, there is no way they can sue you anyway. And even if they did, the chances of them suing you is highly unlikely and way more trouble than it is worth.