Whenever a single miner or a group of miners have access to the majority of the mining power of the hash power of a blockchain, it allows them to get control over the entire network. This is when a 51% attack happens. Given the nature of how blockchain works, a successful attack is the one where a miner must have control over at least 51% of the entire mining pool. In such situations, the attacker or the group of attackers has the biggest hashing power and therefore a monopoly on the mining processes. They then receive the highest mining reward.
51% attack has great potential to disrupt any popular crypto asset. Such assets possess substantial risk from attackers wherein these malicious agents can double spend coins, reverse transactions and leave loads of unconfirmed transactions pending, causing a loss of billions of dollars. Big blockchain networks like Bitcoin or Ethereum are unlikely to ever fall under such attacks due to their highly difficult hash level and high concentration of computational power. These protect the network from getting attacked. Due to the large number of people involved in the mining process of the network, the protection increases.
However, there are a few successful examples of 51% attacks. Ethereum Classic network was attacked in 2019, and Krypton and Shift were attacked in 2016.