Vim mailing list says (re-formatted for better readability):

To change two vertically split
windows to horizonally split

Ctrl-W t Ctrl-W K

Horizontally to vertically:

Ctrl-W t Ctrl-W H

Explanations:

Ctrl-W t     makes the first (topleft) window current
Ctrl-W K     moves the current window to full-width at the very top
Ctrl-W H     moves the current window to full-height at far left

Note that the t is lowercase, and the K and H are uppercase.

Also, with only two windows, it seems like you can drop the Ctrl-W t part because if you’re already in one of only two windows, what’s the point of making it current?

http://vim.wikia.com/wiki/Switch_between_Vim_window_splits_easily
http://stackoverflow.com/questions/1269603/to-switch-from-vertical-split-to-horizontal-split-fast-in-vim

In VIM, take a look at the following to see different alternatives for what you might have done:

:help opening-window

For instance:

Ctrl-W s
Ctrl-W o
Ctrl-W v
Ctrl-W o
Ctrl-W s
...