What is the ruling concerning someone who did not prayed at all for a long time bur started to pray and take religion seriously again without saying the shahada again?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His Slave and Messenger.

If someone abandons prayer out of laziness rather than denial, he is considered a disobedient Muslim according to the majority of scholars' opinion, and not a disbeliever. Most of those who abandon prayer do so out of laziness. The repentance of a person who abandons prayer out of laziness is to start praying, and he is not required to declare shahaadah (the Two Testimonies of Faith) before the prayer, as he continues to be a Muslim. However, he is obligated to make up for the prayers he missed, according to the consensus of the scholars.

On the other hand, if someone intentionally denies the obligation of the prayer, considering it not an obligation, then this constitutes major disbelief, leading one out of the fold of Islam. Such a person can only re-enter Islam by explicitly declaring the Two Testimonies of Faith. Some scholars, however, held that if he prays, he is considered a Muslim. Al-Buhouti, a Hanbali scholar, mentioned in Kashaf al-Qinaa':

“When a disbeliever performs the prayer or calls the Adhan (call to prayer), the ruling is that he is considered to have embraced Islam, whether he was a Muslim before or was an apostate, whether he prays individually or in congregation, whether he is in the lands of Islam or in the lands of war.” Similarly, he is not obligated to make up for the prayers he missed during the period of apostasy, just like an original disbeliever is not required to make up for the prayers missed during the period of disbelief. Ibn Taymiyyah said: “When an apostate embraces Islam, he is not required to make up for the prayers abandoned during the time of apostasy, according to the majority of the scholars. Similarly, a disbeliever who converts to Islam is not obliged to make up for the prayers missed during the period of disbelief, unanimously agreed upon by the scholars. This is the opinion of Maalik, Abu Haneefah, and Ahmad, in the most apparent two narrations from him. In other narrations, the apostate is required to make up for the missed prayers, which is the view of Ash-Shaafi'i, and the first opinion is more famous.”

Allah knows best.