Is it permissible for someone who has stopped praying for days to return to it? And is it better for someone who does not pray at all to remain in that state?

It is forbidden for a Muslim to abandon prayer for a few days, or for certain times, or even for a single instance. This is considered one of the major sins. If those who merely delay their prayer from its appointed time or are neglectful of it are threatened by Allah with woe and destruction, then what is to be thought of one who abandons it for several days? Allah Almighty said: "So woe to those who pray [But] who are heedless of their prayer" [Al-Ma'un: 4-5], and He said: "But there came after them a generation who neglected prayers and pursued desires; so they are going to meet evil" [Maryam: 59]. It is obligatory for a Muslim to observe the prayers, as Allah Almighty said: "Guard strictly the (five obligatory) prayers especially the middle prayer; and stand before Allah with devotion" [Al-Baqarah: 238]. It cannot be said that one who does not pray at all is better than one who prays sometimes, for both are in grave danger.