Hyundai transmission overdrive problem

I have a 2001 Hyundai Accent, 87K miles, automatic transmission. Within the last month, the transmission stopped going into overdrive. This usually happened the first time I drove it that day, and would be fine if I started it later that day or put it in neutral or park and then back into drive. It started doing this once every so often, then once a day, now today it stopped completely. Any ideas?