Bitcoin Hacked From Private Wallet, Can't Figure Out How!?

[MozillianFirefox]

Hello, everyone.

I had deposited 0.06230112 BTC in my personal wallet address:
1Lkt9qtM2nxRtDtGMXbL8M1VduK4FcUZYL
on 27th June, 2018.

I had created the wallet using BitAddress website (offline), copied the private key to a notepad file, created a zip file consisting of that notepad file and securely encrypted the zip (256-bit Encryption) file using an extremely long password. This was done to store my private key safely.

I kept copies of this zip on my mail and whatsapp.

The password to this zip file was stored separately in another email in my mail (I also added other fake passwords in that mail, it didn't look obvious that it was a password to some file).

But suddenly, on 4th December, 2020, someone withdrew the whole balance.

I only came to know of this a few days ago, when I tried to withdraw from this wallet using private key (after unzipping the private key text file using my pasword) and it said insufficient balance.

During the 2 years, I have not done anything to compromise the password to the zip file.

The money lost is an issue, but the bigger issue is I want to know how could this have happened? Please help me figure out how I could have been hacked.
How could anyone have figured out that I have kept my BTC details in such and such fashion, and found the correct password to the zip file somehow and accessed it?!

My mail does not seem to be hacked. Neither does my PC. Neither does my Mobile. Plus, around 4th December, I was busy in my sister's wedding. So, I did not do any activity regarding this wallet.

Also, is there any glimmer of hope left to get my btc back?

[CharlieWex]

I don't want to upset you, but I doubt very much that you'll get your money back.