One of the first things to check when a crossbow will not fire is the safety.
The anti-dry fire mechanism is disengaged due to the spring pushing the arrow up against it. In this situation you may be able to bend a metal spring back in place.
However, if it is broken or if it is plastic you will quite possibly need a replacement.
Trying to test fire the bow without an arrow and finding the anti-dry-fire mechanism remains in place you will need to reset the trigger. Take your rope cocker and pull the string until you hear a “click”. Once you have done that, try and reload and try again.