If you have lost constitution as the result of death, you may purchase it back from one of the NPC healers available throughout the Realm. Healers will remove PvE resurrection sickness at no charge, by right-clicking on them. If you need to have constitution points restored, right-clicking again will prompt you with a popup window informing you of the cost of the donation required. If you accept it, the healer will restore as much constitution as you have money for. Note that if you don't have enough money to restore a constitution point, a healer will continue to prompt you every time you right-click on them. If your constitution is not being restored, this is likely why.