06 hyundai elantra wont shift into 5th gear

When driving on the freeway, my hyundai elantra won't shift into 5th gear. I sit at about
3000rpm at 75mhp. I took a trip to california last year, and did 85mhp at 2300rpm almost
the whole way. Its just been the past couple weeks, and the car is really well taken care
of. I bought it from my grandma, last winter at 26000 miles and she was the first owner.
So I'm kinda suprised that its having these problems. Is this a common problem, and is it
a simple fix? Also reverse works fine and it drives perfectly except at freeway speeds.