My Bitcoin transaction is lost
the Blockchain says that the transaction is "Spent" What does this mean?


It means that the outputs of the transaction have been used as inputs in a further transaction.

Which, in simpler terms, means that the recipient has subsequently spent the received money.

If you were the recipient, have a conventional (non-custodial) wallet and didn't knowingly spend the money it probably means you have lost control of your wallet as a result of hacking or social engineering or some other hostile act by a third party. For example where an online "friend" helped set up your wallet or where you clicked an "update" notice in an old Electrum wallet.