Maybe at some point you cleared your cookies (or they got cleared) from the browser you use to access the forum. This means that if you want the forum to remember you each time you visit, do as Lazie has suggested and check that "Remember me" box next time you login.
If you performed some maintenance and your cookies get cleared again, you can expect to experience the same thing until you check that box again.
“Only when you drink from the river of silence shall you indeed sing. And when you have reached the mountain top, then you shall begin to climb. And when the earth shall claim your limbs, then shall you truly dance.” ~ Kahlil Gibran