If a pool is properly winterized, it can easily be reopened come swimming season. Most importantly, don't remove the pool cover until you've cleaned the area around the pool. Sweep or hose away debris to prevent it from getting into the pool. Next, use a garden hose to fill the pool to its normal water level. Reconnect everything that was disconnected. Water will need to flow through the circulation system, so open the skimmer line valve. Test the water for its pH level, then shock the pool. It'll take a week or more before the pool gets balanced and becomes swimmable. Leave the pump running 24 hours a day, and reduce the run by only an hour or two each day until the water is balanced.