All of a sudden = go get him checked by a vet
I can see only two reasons your dog would suddenly refuse to climb the stairs: he got hurt and it is painful for him to walk up the stairs or he had a negative experience and learned to be afraid of the stairs (or a combination of both: he got hurt in the stairs, that's the negative experience and now it's painful as well).
In any case it is worth checking with your vet.
Dragging him with a leash = animal abuse
Why would you do that?
If he's scared you're just reinforcing that fear and you're making the stair and your presence a negative experience.
If he's got a physical problem (might be temporary, a sore muscle for example) you're probably hurting him even more, you're creating a very negative experience and that means that even if its physical problem is temporary, he'll learn to avoid the stairs even when he'll get better.
Of course I can understand that at some point you might need to take him upstairs. Why don't you just carry him up the stairs in your arms? It is definitely possible to do that with a German Shortair Pointer (according to Wikipedia the weight of a male is around 25 kg). If he doesn't let you do that, train him to like it.
Away from the stairs, lift him up and give him some treat, slowly increase the duration and treat.
As for everything training is the key
Now that you had him checked by your vet and that you stopped making things worse, you can start helping him to be confortable with the stairs again.
That kind of training is called counter-conditioning.
The idea is simple but it requires a lot of patience. You simply reward every (tiny) steps taken in the correct direction. If you get your dog interested in some treats, attract his attention and move closer to the stairs, reward him for following you and approaching the stairs. When he won't approach closer, throw a treat away from the stairs, and reward him for coming back, do that little game for a while, and then reward only when he takes one more step. You get the idea.
You can also do the same game elsewhere were he'll agree to climb the stairs. That way you reinforce his efforts and you set him up for success.
At the same time I would suggest to reinforce him for being upstairs. Make upstairs be his favorite place to stay.
You can also reinforce him when you take him up the stairs.
With patience and a combination of these exercices you'll start to see improvements.