I bought my first few Bitcoins in 2012; eventually I got around 100 and managed to hold most of them until this point. Yesterday they were stolen from me. Shortly after I made a transaction, I left my PC with Electrum open, I believe there was a key logger waiting for me to type my password as when I returned an hour later, I saw my account had been emptied......
There are other possibilities.
A) you had downloaded, unwittingly an infected copy of Electrum
B) someone walked by your computer, saw the open program and took what they saw