Question

how did time turn ozymandias to nothing

Answer 1

The time damaged and destroyed the great works of ozymandias and also his statue
Due to powerful time, nothing remained besides two vast, trunkless legs with broken visage lying around half sunk and vast stretch of sand........

Answer 2

this story tells us about an old, broken statue in the middle of the desert.
this sculpture was once a symbol of pride and greatness, but now, it is no more than broken stone.
time has kept no other evidence of his strength in the nearby of his giant and broken statue.