My 2001 Toyota Camry Le 4 Cyl Automatic Transmission

My 2001 Toyota Camry 4 cyl automatic transmission has started giving me problems. The car has over 200,000 miles on it, however, it has always had regular oil changes. Anyway, the timing belt snapped and my husband replaced it with a new one. (No easy task, let me tell you). The car started up in Park easily and sounded wonderful. However, once I put it into drive, I felt it slip. Now, the car will only engage in reverse, and park/nuetral. It will allow me to move the shifter to the forward gears, but the car only revs up the RPM's and will not engage in drive, second or low at all. We had a diagnoistic unit put on it and the only reading we were able to get said the computer had no codes stored. The battery was disconnected while the timing belt was being replaced. Is it possible that the codes were reset because of the disconnected battery?We checked the fluid and it was low, so we added some and rechecked the level. They were good. We have also tried to do all of the tests we found in Chilton's. We tested the shift cable, the park/neutral switch, and the throttle cable. Our next test will be to change/flush the transmission fluid in both the transaxle and the differential and if that doesnt help, we will try the manual shifting test by disconnecting the solenoid. And before taking it to a repair shop, we will try using the OHM meter to test the solenoids..I have had no problems with the transmission ever in the past. What is the most likely diagnosis? And are there any other do-it-yourself tests to determine the exact problem OR is it possible to get any old codes?I appreciate any and all help.