Quick answer:  I know, if they won't eat with a muzzle it is very stressful for both of you. Does the muzzle fit?  How long have you tried? One day is probably fair but you won't stand it much longer. Here's a few tips:

What kind of muzzle is it?  The basket type ones often grasp the horse's face a bit too much, they don't move and a horse can feel trapped and not even interested in eating as 'trapped'  trumps hungry.

What kind of grazing hole is there?  Almost all grazing muzzles have a round hole and this is difficult for the horse to get grass through. If they are willing to keep on trying, many will get the hang of it and find grass.

However some just won't.  In which case you likely need a different kind of muzzle.

See the LiteBite:  1.  Soft and moveable   2.  Roomy and airy   3.  Most of all it has a unique grazing slot which allows for a full bite not a nibble through a round hole.

This give your horse the idea very quickly and even though they don't get much through the grazing slot, it will feel as if they are grazing normally with that full bite.