Cant close upper and lower teeth due to a badly fitted or too large mouth guard?

What. What are you waiting for? Get back to your Dentist ASAP. Sometimes mouth guard needs adjustment. Sometimes occlusal orthotic Corrects your jaw position and teeth then have to be adjusted or straightened to match. A consultation with an Orthodontic Specialist may be in order.
Possible. See an oral surgeon or someone who handles tmd probles soon. If this just happened it can probably be reduced and repaired with little problem. If one waits too long the tissues stretch and reduction is less successful.
See your dentist. Mouth guards can be fabricated with different materials and can vary in thickness. They will prevent your upper and lower teeth from contacting while wearing it. If you feel it is too big or not fitting correctly, you should return to your dentist who placed you in the mouth guard to see if you need a different appliance.