Climate problem Stilo Bj. 02 – just does not get cold (longer service life)

Hi together. Now I came out of the holiday happy tonight and put my Stilo in the blazing sun this morning. Must say, he stood about 1.5 weeks in the garage and before he was driven very little in the last 5 weeks (250 km). Now I wanted to turn on the air conditioner today, but no colder air comes than if I only let the blower run. So thought of either a defect of the climate, a drop of the electronics, it had at least 45 – 50 degrees in the car o The latter I rather exclude, because in autumn 04 a inspection was made and because the climate maintenance was carried out and about 800 ml was refilled. (I think the refrigerant was completely changed and there was probably only 300 ml missing the master said, disinfection was also done immediately) What do you advise me? Does it come from the service life? is too little refrigerant already in again – where can I check it (possibly do it yourself)? Thank you!