Bitcoin transactions are generally not reversible. If this transaction has been confirmed, your coins are gone.

If you think the transaction was erroneous, you can contact customer support for your wallet provider. If they discover that it was due to a bug in their system, they might reimburse your money.

Unfortunately, a common cause of such loss is that someone compromised your account, e.g. by guessing your passwords, or by hacking a computer that you used to log in to that account. If this is the case, it's unlikely that you will be reimbursed, and all you can do is plan to improve your security in the future.

In either case, it would be unwise to use this wallet ever again. If there are still funds there, transfer them immediately to a different wallet; ideally, one created using a freshly installed operating system on a secure computer.