If you happened to hit I, and not give a replacement, it would remove all instances of the letter I.

Possibly you inadvertently hit the shortcut for said tool.

able to be very specific. Odds are you accidentally pressed the Ctrl or Alt key along with some other letter, like, say R. That could have called up a Replace routine, which displayed the little box you didn't pay attention to. Apparently, you accidentally replaced the letter "i" with nothing.

Aside from mentioning that boxes that pop up in applications should be checked carefully, I don't have a lot of offer you, and you had to go back and retype all of those "i" letters.

Again, not knowing what application you are using, you might have been able to undo the action that replaced all those "i" letters with nothing by pressing Ctrl-Z. That's pretty much the universal keystroke for Undo.

It happens. The most common, most disastrous error is pressing Ctrl-A by accident, which selects all of the text in the current window. Then, when you press another key, all of that text gets replaced by whatever character you pressed. Ctrl-Z fixes that, too, in most text editing applications.

I know that doesn't help solve that problem you had, but it sounds like you fixed it yourself. But, do read pop-up boxes. They're often important.

...and an electronic medical records system called EPIC. Windows software.

Just to be safe.

Lenovo ThinkPad and Windows. The electronic medical record system is called EPIC. No choices in any of that.

My condolences.
And apology for not being helpful.

Have you posted any anti-Russian posts on social media?

but now it's EP C?

Sorry, bored at the office.