When you type commands, text jumps around on the screen and nothing works the way it's supposed to.
You may have hit the
[CAPS LOCK]
key without noticing it.
vi
is case-sensitive. That is, uppercase commands (
I
,
A
,
J
, etc.) are different from lowercase commands (
i
,
a
,
j
), so all your commands are being interpreted not as lowercase but as uppercase commands. Press the
[CAPS LOCK]
key again to return to lowercase, then type either
U
to restore the last line changed or
u
to undo the last command. You'll probably also have to do some additional editing to fully restore the garbled part of your file.