You're smart and don't keep your digital money on an exchange; it's still not safe unless locked down.
When using an exchange, it's three confirmations and out!
You're off to a great start!
But unless your precious bitcoins are locked away on a Ledger, Trezor, KeepKey or Digital Bitbox hardware wallet then they may all be gone one day - even if you keep them on a "Core" client or use a wallet with the backup phrase.
Hackers are smart
This post, from reference image above, was only a few months ago on the bitcointalk.org forums. He had all of his BTC stored away on an SPV wallet and he knew his wallet words. The computer was on and connected to the Internet, but the Bitcoin wallet was not running.
This all happened when Bitcoin was making daily all-time-highs for the first time since 2013. He decided to enjoy some of the profit and went for a weekend trip. He came home, opened his wallet and saw somebody had sent all 87 BTC!
After some discussion, there was consensus that he had been phished with a key-logger.
The chances are that you had a keylogger - a type of malware that many antivirus programs can't detect. It monitors everything you type so that the virus can find details about bank accounts, potentially e-mail accounts, and other sensitive information. I would suggest that you instantly factory reset your computer, set up a new operating system and more. DO NOT ACCESS YOUR BANK ACCOUNTS. The keylogger may have been monitoring your activity for weeks before to pull this off.
Hardware fails and so does software
I recently did a fresh install on my iMac. It'd been a couple of years and things were starting to get a little buggy. I made a couple different back-ups to my wallet. I'd moved everything to Jaxx, but felt really uncomfortable with the large sum on a hot wallet.
I syned Core to the Bitcoin and Litecoin blockchains. I moved my funds and began setting up my Trezor. The Ethereum Classic was taking longer than expected and the Litecoin wallet needed upgrading for SegWit. It took about thirty minutes.
Meanwhile, auto-update was running in the background. I ignored it like normal. Something had been updated and it wan't to restart. I just wanted my Bitcoin and Litecoin onto my new Trezor. I went back to my Core wallets and saw this:
Get your hardware wallet!
You will save more money than the price of the wallet. I, personally, go directly to the manufacturer, unless you want to pay @bitcoinmeister to walk you through erasing the existing software by flashing the firmware.
There are other reputable firms out there that sell crypto hardware wallets and accessories. If you buy second-hand, have it flashed. Otherwise buy a new one direct, check the security seal and protect your bitcoins!