Stuck in 3rd

Earlier I was trying to locate the transmission dipstick of my 1972 340 Duster. The reason is because I put it in the shop and had the power steering repaired and a new gas tank installed. When I got the car back after the repairs were made it drove beautifully. It drove so smooth that I spent most the day driving it just because it was fun. When I finally got home with it, as I'm turning into my driveway I attempted to downshift and couldn't get it out of 3rd gear. I figured maybe it needed transmission fluid. It's amazing to me that the thing shifted so nicely all the way up until I get it home but then that's my kinda luck.

Is this a common problem and what might the cause be?

Thanks in advance.