Right.
Have any martial arts gone extinct? Far more than exist now, to say the least. Some arts only last a single generation, with no-one continuing them for many reasons. Others last longer, but not forever. These include arts from everywhere, such as many forms of Japanese arts, many Chinese systems, and many Western systems (such as traditional wrestling forms, pancration, and many more), as well as all Korean systems (albeit with some controversy... but nothing that currently exists as a Korean system is what was around way back when).
How about in the last 50 years? Sure, of course there have been. I train with a gentleman who learnt a particular form of Chinese martial arts which ceased to exist when his teacher stopped teaching about 10 years ago. If no-one is in a position to continue a system, it dies.
Can you resurrect them? Well, that's the question... I would say yes, provided there are certain criteria met. First, there needs to be a closely related system still extant to be used as a reference point. Next, the person reconstructing the system (that's the common term, by the way) needs to have experience in similar or related systems. And thirdly, there needs to be enough information left to base the reconstruction off. Reconstructing arts happens a bit in Japanese traditions, primarily due to the large amount of documentation that remains of old systems. In recent times, arts such as Katayama Hoki Ryu Jujutsu have been reconstructed (70 years after it went "extinct"), and sections of other systems such as the newly created Betsuden Mokuroku of Toda-ha Buko Ryu (containing it's methods for Kusarigama and Nagamaki, reconstructed from old documents the Ryu maintained).