There are several ways you have lost your BTC,and the one that is most likely is that you download Electrum from fake site.At that time there is many fake Electrum sites shown at the top of search results and if you not careful and check site you got fake Electrum.The only legitimate site for download Electrum BTC is https://electrum.org/#home

Other way is that you have some RAT(remove access trojan) on your device,so hacker is get your private keys/seed.Electrum is completely safe if it is download from official site and if user device is clean from virus/malware.