I recently installed Ubuntu alongside Windows 10 on my Thinkpad. However, some idiot (who didn't know my password) accessed my computer while I was gone (after I hibernated it) and must have messed with something because when I got back and opened my laptop, I had to type in my Bitlocker recovery key, and even after disabling/reenabling Bitlocker, I still had to type in the recovery key every time I restarted the computer (I eventually had to decrypt and reencrypt the SSD to fix this.)
Now, instead of showing me an option to boot Ubuntu or Windows, it just goes directly to Windows (after the Thinkpad BIOS screen, of course)
This is really weird considering that all the guides on the Internet for uninstalling Ubuntu (that I could find) required a Windows Recovery disc or logging into Windows.
I know that this person had access to no equipment other than the ThinkPad in question and a smartphone with Internet access. Guest account was disabled in Windows, and I doubt he was smart enough to use guest account in Ubuntu (still a possibility though).
So how did he manage to disable the Ubuntu bootloader without a recovery disc when not even the Internet knows how?
UPDATE: It seems that I can't boot into Ubuntu by pressing F12 to select a temporary startup disk.
Keep in mind that this person is not well-versed with computers and didn't have access to any equipment other than an Android phone with Internet access, so he couldn't have opened up the computer or booted up a special OS on a USB stick.
UPDATE 2: Okay, apparently he figured out how to download a VPN app on his Android phone even though we're in China right now (normally you need a VPN to download anything on the Play store since China blocks Google). That's quite impressive. Still, he tried using my computer (probably to browse the web) even though he doesn't know my password (forgot if guest access was enabled though) and at first didn't know what to do when he saw the Ubuntu bootloader, hence why I assumed he was an "idiot"
Note that this guy probably just wanted to use my computer to browse the web, not intentionally disable it or steal anything.
UPDATE 3: No, it probably wasn't a Windows Update or a Lenovo update since none were installed recently. Besides, this happened after I put it in Hibernate mode.
UPDATE 4: Okay, it looks like he could have also used my Chromebook, but only in Guest mode, but it seems irrelevant since I don't see any way it could have been connected to my Thinkpad.