2001 Jeep GC transmission problems

I have a 6 cyl. 2001 Jeep GC with 80,000 miles and a problem. Jeep was running fine yesterday and today I put it in Drive and the transmission would never shift into 2nd, it would only stay in 1st. Reverse works. Checked the oil and color and smell were OK. Does anyone have an idea of what problem may be.

Thanks