You can leave Canada and come back. However, one of three things will happen when you return to Canada.

1. You may be allowed to come back to Canada as a visitor, if Citizenship and Immigration Canada (CIC) has not yet decided to extend your work permit. If this is the case, you cannot work until you get your work permit extension. The officer at the port of entry may ask you to prove you have enough money to support yourself in Canada.
2. You may be allowed to come back to Canada as a worker, if the officer at the port of entry finds out that CIC extended your work permit while you were away.
3. You may be asked to apply for a new work permit at the port of entry.

You must also meet all other applicable entry requirements.