Blockages are the most common reason for sprayer failure. Remove, check and clean the following parts to find blockage: nozzle, trigger, lance, hose and pickup tube.
This could be due to pump not being screwed on correctly, or safety valve being lifted. If this is OK, then check hose and tank for damage.
Check the mono valve at the bottom of the pump cylinder for wear or damage. Check the o-rings inside the cylinder for wear and damage, and lubricate before use.