Safari > Private Browsing - is there a check next to it?
If so, then I think your computer automatically deleted your browser history. Uncheck Private Browsing if you want your history to be saved. Otherwise, the history was deleted somehow, probably by slip of the finger.
There might be a way to access the history through your system library, but I'm not sure. Perhaps someone else can help. Hope this helped you.
Unless you've done a backup lately, that information is, unfortunately, gone. BTW, private browsing won't delete prior history, it just won't add to it while in that mode.