1997 Honda Accord Transmission issue

The check engine light came on about 10 miles from home. Drove about 8 miles to get home. Before problem occurred. Engine revved but the car acted as if it was out of gear. It would not run in drive 4, would in all other gears and reverse. Would not go over 20 mph. No prior indicators of transmission problems, no sliipage, going in and out of gear, no grinding, etc. Repair shop though it might be a sensor issue that restricts systems to prevent major damage. Now they say it needs an expensive transmission, but they have not put in a new sensor to see. Could it be a sensor issue or more likely a new transmission issue?