It is possible that you have been a victim of hacking or phishing.
It is possible that somehow, your device has become compromised and your wallet is therefore vulnerable to theft.
The first thing you should do is create a new wallet and transfer all remaining funds to the new wallet.
Install a good antivirus software and clear your device of any and all malware and viruses.
It is extremely important that you store your login information off of your devices entirely. You mnemonic seed and private wallet keys should not be stored digitally, as these can easily be accessed if or when your device is hacked.
Another important things to note is when you navigate to MyMonero.com, be sure to type out the entire address manually, as phishing sites can use autofill suggestions with minor url differences and this leads you to enter sites are not ours.
And once you type in your information on a phishing site, your information is made available to them, which means your funds are made available to them too.
Sadly, if there have already been transactions made out of your wallet without your consent, these cannot be traced or reversed.
Monero is a completely untraceable currency and this has its pros and cons.
The only thing you can do is ensure that this does not happen again.