Once the samurai lost his master he becomes a rōnin. Could he become a samurai again by acquiring another master?
The samurai used to change their masters a lot, especially in the Sengoku era. A rōnin who came from a defeated clan where his master has been killed can attach himself to another clan and serve as a samurai.
In the movie Seven Samurai, several farmers hire rōnin to protect their farms from bandits. the movie describe them as noble heroes who stand up for oppressed farmers. the reality is quite different than that was portrayed in most films.
So we can see in the above example that a rōnin can still protect and fight.
In the battle of sekigahara in 1600 a lot of samurai became rōnin, most of them lost their masters, around 500,000 rōnin existed without income. At the era of tokugawa shoguns, those rōnin divided into two rebellions. The first, led by Yui Shosetsu, was aborted before the actual attack. And the second rebellion was unsuccessful. Tokugawa at the beginning and at the end of 17th century engaged in a campaign of suppression and advising the masters not allowing entering those rōnin to enter their fiefs.
So yes, a rōnin can have another masters, if the master needs or wants a rōnin.