Ancient Greece never really declined. But it did fall.
Historians refer to Ancient Greece as a civilization. That's because it was never an empire. It was never a country. (Greece did not become an independent country until modern times, in 1821, or less than 200 years ago.) Ancient Greece was a collection of independent city-states with a common culture. Most historians agree the Greek culture was a foundation culture of Western Civilization, which means a root or a beginning. There is no doubt that the ancient Greek civilization has been immensely influential on language, literature, educational systems, philosophy, art and architecture, politics, theatre, drama, science, medicine, and mathematics.
The time period called Ancient Greece is considered by some historians to begin with the Greek Dark Ages around 1100 BC (the Dorians) and end when Rome conquered Greece in 146 BC. Other historians start with the 776 BC Greek Olympic Games, after ancient Greece had formed themselves into hundreds of independent Greek city-states, each with their own way of doing things. It's safe to say that the ancient Greek culture covered a lot of years. Ancient Greece was at its pinnacle from 776 BC to 146 BC. For a very short period of time, within that pinnacle, the ancient Greek city-states were pulled together under one rule - not their own rule, but the rule of Alexander the Great.
Alexander the Great conquered the ancient Greek city-states in 338 BC. Alexander ruled for about 12 years. Alexander died young. He was only 32 years old. He was off conquering other lands when he died. During his rule, daily life and religion in ancient Greece continued much the same as always. Alexander greatly admired the Greek culture. He was not a Greek; he was the king of ancient Macedonia, a country to the north of the Greek city-states. If there was an uprising against his rule, he would squelch it (as in level.) He left the people he conquered free to enjoy their own culture, as long as they were loyal to Alexander.
Alexander was a military genius. He did not want to sit on a throne. He was a conqueror. He took his army and conquered all the civilizations around the Mediterranean. He pushed on as far as India. He never lost a battle, not even once, but of course he did lose men. He swelled the ranks of his army by taking men from the various civilizations he conquered, including men from ancient Greece. Everywhere he went, he spread the Greek culture. He probably would have continued on to China, but he died of an insect bite, while standing in a swamp, or so it was reported. There was a rumor that his men had killed him. Whatever the truth, after his death, his army fell apart. His men returned to their homes.
The ancient Romans: About two hundred years later, in 146 BC, the ancient Romans attacked the ancient Greeks at the Battle of Corinth. The Romans won. But the Romans loved the Greek culture, especially the Greek gods and Greek myths. The Romans adopted all the gods and all the myths, changing them a bit to reflect the Roman way of life. As long as the ancient Greeks agreed to consider Rome in charge, the Greeks were free to mostly manage themselves. Even their language remained the same. Once again, the ancient Greek culture survived. In fact, it expanded - as the Romans expanded into Europe, they brought with them the Greek culture, which they claimed was the Roman culture. (The Romans often did that - adopt something, and then pretend it was Roman all along.)
So, Alexander spread the Greek culture around the Mediterranean, and the Romans spread the Greek culture into Europe. Greek culture is still influential today. That's why historians say Ancient Greece never really declined. But it did fall.