96 Jimmy No Forward gears

Hi, I have a 1996 GMC Jimmy with a 4.3 4x4 automatic. I was driving, and came to a stop sign, stopped then went to take off and had no forward gears. Tried each one ( 1st, 2nd, 3rd and 4th) and had nothing. Reverse works just fine. I have had no problems or hints of any thing wrong. It has about 140,000 miles on it. Is this possibly just a solenoid or valve body issue or could it be worse. Any help would be great.

Is the SES, (Service Engine Soon) light on? If so, what codes are stored? When was the transmission fluid and filter last changed? Should be done every 30,000 miles.