My horse is ten years old and healthy most of time. The case is very surprising and strange that he was healthy last night before he became a lame the next morning, and his hock catches when it walks.

48northfarm is absolutely right: you need to call your vet now.  Allowing lameness to go uncared for--even if it is only minor to begin with--can cause serious damage to the leg as the horse moves around, and you may end up spending more on vet bills than you would have if you had called right away.  Take it from someone with a little experience: Expensive is cheap in the horse world (and in most other areas of life, as well).  You'll either pay a little for it now or a lot for it later.