When this happens, the first place to look is under the lid to see if the product pouch is empty. With the lid in hand, you're able to confirm if the roller is installed onto the lever. If those two things looks good, deeper investigation is necessary.
Gently remove the pump and attached pouch from the vessel. Check to make sure the pump dome is installed completely - that means all the lines around the dome edges are within the pump housing. If all your moving parts look good, it's time to check the connection of the pump to the pouch.
Is the pouch fully pierced? A good way to be sure is to push the pump connecter piece further into the pouch fitment. If you hear a snap or click, it's in place now and you should be back in business.