My ETH funds were stolen and I am sure I didn't give out or handled carelessly the Mnemonic phrase, JSON file, private key nor the key store. Please I want to know ways by which a cryptocurrency wallet could be hacked.
What are the websites that you visited recently? My suspect is you just became careless and you logged in to a phishing link though you said that you are careful of not giving out your keys, you are still vulnerable to log in to a fake site / phishing site that you aren't aware if you are in a hurry or you just really don't care of that site you are logging in. Like what has been said, you probably downloaded something that contains malware that affected your computer that allowed the intruders to touch your files on your pc.
This is the most likely option, there are many people that have seen movies and have distorted ideas about hackers, without a doubt they know a lot about computers but most of the time they prefer to deceive people than to deceive computers since it is way easier to deceive people, so most likely the OP was sent a phishing link and he accessed it thinking it was a link to the original site.
Well, even those phishing site sometimes can get into google adwords overtaking the position of the real official website to #1 in the google search engine mainpage. I don't really recommend using any kind of web wallet unless you are really aware of what risks you may get exposed to.
Just stick with desktop wallet that has 2fa and everything will be fine, just imagine if you tighten the security earlier you won't lose that money.