What Happens in a 51% Attack?

Once attackers control the majority of the network’s power, they can manipulate the blockchain ledger in ways that are normally impossible.
They can spend coins, then rewrite the chain to erase that transaction, effectively spending the same coins again.
They can block or delay new transactions from being confirmed.
They can replace parts of the blockchain with their own version, invalidating recent blocks.

What They Cannot Do
They cannot steal coins from other wallets.
They cannot change old transactions that are already deeply embedded in the chain.
They cannot create new coins out of thin air (that would break consensus rules).

Proof-of-Work chains (like #Bitcoin, # Monero, #Ethereum Classic) are vulnerable if attackers control enough mining power.
Smaller blockchains with less total hashrate are more vulnerable, since renting or redirecting enough power is easier.
Larger networks like #BTC are considered safe because the cost of acquiring over 50% of hashrate is astronomical (billions of dollars in hardware + electricity).