Okay...we have a 2 year old border collie mix. When we first got him, he would not climb our stairs. Then we sort of cradled him up the first two steps and he took off. Then with absolutely no problems would bolt upstairs to climb in bed with my wife for months on end. Then one day out of the blue he stopped climbing the stairs and is petrified again. I've tried to teach him as before with no luck. Any ideas?? He's had no accidents on the stairs that I'm aware of.
Have you tried bribery?? Does he love his dinner? Maybe one night dinner is served on the bottom step, the second night on the second, and so on... or just get some yummy treats and do the Hanzel & Gretel thing... give him a trail of hot dogs to follow or something. Are the stairs bare wood (slippery)? Maybe he fell when no one was around and just needs to regain confidence. See if you can get him to slow down - I know that's tough for a Border Collie!