How cold did it get in the greenhouse? Some hoyas are more cold sensitive than others. Most can take some filtered sunlight ok. Was the sun shinning right on it after moving from a shady place? What was the watering schedule? Did it go dry too long before watering? Many things can cause a hoya to go downhill. Best to get some cuttings from it now in case it doesn't recover.

Hi, there is around 7 C° only in the night, I water around 1 time every two week from 2 months but I didn't understand if it were for the cold o for the sun....in this period we don't have an hot sun.

Does the whole plant look like that, or is it just one piece or side? If it's just one part, just take those vines off. That's colder than many Hoya like to be, but some common ones do survive fairly low nighttime temps. You can take it out and look at the roots. If the roots are all brown and crumbly, then the plant will need to be restarted from cuttings or just lost. Sometimes we just can't know what made it happen. Sorry.