How do I get my Overdrive back on on my 98 Mazda 626?

I just noticed it was off this afternoon and the light was steady on my dashboard that said "O/D off". I know some of these cars have transmission issues and I also noticed that when I was steadily doing 45 going uphill, downhill, and on a straightaway the car was at 3 RPMs when usually it shifts on or before that.. How do i get that light off in my car?