There are several problems with answering that:
You say you don't like helping your teammates because your tower is unprotected. That means you don't think enough about lane dynamics yet. As AP mid, the way you play is basically you push the lane to the enemy tower and go do other stuff (gank other lanes, take a jungle camp, take blue, etc.). You can do that freely because the lane is pushed, the opponent can't destroy your tower since he has to push the lane back first which takes about 30 seconds to a minute until the minions are at your tower, depending on how strong you pushed the lane and how strong the opponents champion can push.
That is enough time to go to another lane and help them.
If you destroyed the first tower, you can push the lane even further back, giving you even more time to gank other lanes.
Generally speaking, once you destroyed the tower or gained the ability to insta-clear a minion wave, you should spend as little time as possible in your lane and as much time as possible ganking other lanes and then be there just in time to catch the big counterwave coming in range of your tower. In the meantime take your and sometimes even his wraith camp for some bonus farm while you go to top or bottom lane.
Another way to approach this situation with more defensive mids is to freeze the lane, that means let the lane come to you and tank it far enough away from your tower that it doesn't shoot but close enough that the enemy mid can't farm your minions. Then farm very, very slowly which denies the opponent his farm since he can't reach your minions safely and since he has no tower to back him up, it makes him extremely vulnerable to ganks which can snowball your advantage even further. However, defensive mids will rarely get into this situation in the first place.
However, the situation most likely means that the opposing team is just bad, so you could do whatever and still win. It should never occur in a match of equally strong teams of decent skill.