It depends how long you left it dormant... if it happened within the last 90 days, your minutes and/or data boosters will still be there in full. All you need to do is top up your base plan and you're good to go!
However, if you left it longer than 90 days, I'm afraid your account + number were canceled, and your boosters expired. You'd need to buy a new SIM and pick a new number... but you'd also receive a $20 credit again to get you going :)
If you are interested in just selling the phone, the new owner can just buy a SIM card for it (or even go on a Koodo plan, if they so desire) :)