A ronin is simply an unemployed samurai (i.e., a samurai without a position as retainer to a lord). If they obtain a position as a retainer, they cease being ronin. If they lose their positions, they become ronin. Whether ronin or retainer, they were still samurai.

In principle, being a ronin was a temporary state. In practice, it was often permanent, or only ceased when the ronin gave up his samurai status. For some - Edo Period samurai who were fired by their lord - it was meant to be permanent; they were forbidden to become retainers to another lord, and it was illegal for daimyo to employ them as retainers.

The main problem for ronin was that as samurai without positions as retainers, they were without the stipends that came with those positions. Unemployment meant no regular pay. If they could find a new position quickly enough, all would be well. If not, they could starve to death (and some did), or they could earn money as craftsmen, as guards, as mercenaries (including outside Japan), as martial arts instructors, as teachers, or become farmers, or bandits or pirates. Those who became farmers or craftsmen often gave up their samurai status (Edo Period samurai were not allowed to engage in such work, so giving up their samurai status was a way to make their work legal).

The heyday of the ronin was the end of the Sengoku Jidai and the first half-century of the Edo Period. As Japan was being unified, many daimyo and their fiefs were destroyed, their samurai becoming ronin. In the early Edo Period, there were perhaps 500,000 ronin in Japan. While the country was still at war, there was still demand for warriors, and ronin could join one side or another in a battle, and hope for a position as a retainer if (a) their side won the battle, and (b) they distinguished themselves in battle. With peace, there was less demand for warriors, and ronin often stayed ronin until their deaths. Their deaths (from old age, starvation, and battle - many ronin fought on the losing side in the Siege of Osaka Castle in 1614-1615 and rebellions against the shogunate) and giving up samurai status reduced this number, and the "ronin problem" stopped being such a huge problem. However, it didn't go away, and ronin continued to be a problem for local law and order and the shogunate.

In the peace of the Edo Period, ronin could still become retainers. Talent was one path to a position as a retainer. Another path was purchase/bribery - illegal, but practiced. Indeed, commoners could obtain samurai status, possibly including a position as a retainer, by purchase/bribery. As one samurai, Buyō Inshi, wrote,

Some save up a sizable fortune, bid farewell to their hereditary lord, and temporarily become rōnin. They then use their savings to purchase a position as shogunal houseman. Others do the same thing: the sons of unpedigreed townspeople; those who greedily collect immoral amounts of interest by lending money at extortionate rates; the sons of blind moneylenders; and people from distant parts who have performed evil deeds in their own localities and, unable to live there any longer, turn their backs on their parents and head for Edo. Among those who obtain such shogunal housemen posts, are there not men who, having incurred the displeasure of the domain or fief holder who was their original lord, were released from his service with a ban on taking up service elsewhere? There are bound to be as well monks laicized for having breached Buddhist precepts. Perhaps there are children, too, of monks bound to adhere strictly to the precept [of celibacy]. There may also well be, further down the order, pariahs and outcasts and their ilk. The reason for this is that the only thing held to matter is money, and not the caliber of the person.

He wrote about the problems faced by ronin - poverty, unemployment, starvation - and the problems that resulted for wider society, such as violence, crime, and corruption. A major root of the problem, as he saw it, was the change from society being run by warriors to society being run by money, and the problems of and by ronin were symptoms of this underlying social disorder.

Reference:

Buyō Inshi, Lust, Commerce, and Corruption: An Account of What I Have Seen and Heard, by an Edo Samurai, edited by Mark Teeuwen, et al., Columbia University Press, 2014.