OneNote allows you to password protect and encrypt an entire section, so you could make a journal section an store all of your entries in there. It works quite well from my experience. Evernote also offers encryption but you can only encrypt a selection of text and it can be inconvenient to have to encrypt each entry separately. Both options have mobile/desktop/web apps.
Another option is a private Wordpress journal.
A lot of people here recommend using a physical journal and a pen (I'm planning on going this route eventually). You could also use good ol' text files synced with dropbox (or something else) and encrypt them yourself.