I feel empty, that my Kavanah is lost, cuz cannot pray like before. concentration is lost like before..... please advise how to develop Kavanah in prayings.

Shalom, Thank you for your question. Praying with intent (kavanah) is a very difficult task. It is not something that a person learns then has – such as a skill in riding a bike. Rather it is a task that requires daily renewed efforts. Even if someone merits to pray today with kavana, that is no guarantee that will have it tomorrow. So, what you are feeling is quite normal – don't despair. As to what to do to attain kavana – there are three major steps. Firstly to try an be aware that one is standing before G-d and talking to Him (this is harder than it sounds, but with effort, and using one's imaginative skills, it can be done). Next, to try to focus on the meaning of the words. To do this, one will need to learn both the simple meaning of the text (or a translation from the Hebrew), and the deeper meaning of the ideas. For this it is a good idea to set aside time to study any of the many good books written as commentaries to the Siddur. Lastly, one should try to put their heart into their pray – and talk to Hashem as to what is important to you – such as not being able to pray with kavanah! This personal heartfelt cry to Hashem will certainly help you along the path to prayer with kavana. Blessings.