Just as the 1970s were beginning, Marvel released X-Men #66 (by Roy Thomas, Sal Buscema and Sam Grainger). In it, the X-Men dealt with the shocking return of Professor X the previous story (who they had all believed was dead, but was secretly keeping himself in isolation so that he could prepare to defeat an impending alien invasion) while suddenly having to fight the Incredible Hulk. It was a fine issue, but certainly not something that would normally stand out.
However, there was something very significant about this particular comic - it would be the last new X-Men comic book for over five years. Yes, as 1970 began, Marvel was beginning to transition a few of their lower-selling titles to reprint-only status and X-men was one of them.
The next issue, X-Men #67, reprinted an early Stan Lee/Jack Kirby story involving the Juggernaut. Now, do note that back in the early 1970s, back issues were only recently becoming a real "thing," so if you wanted to see the classic stories you knew that a company had done in the past, the only way to do so was through reprint series. This is why Marvel had a robust reprint department for the rest of the 1970s. When they began to cut down on their reprint books, they were publishing as many as six comic book series devoted solely to reprinting older Marvel comic book stories (by the 1990s, it was just Marvel Tales, which repritned Spider-Man stories and X-Men Classic, which reprinted Claremont X-Men stories - neither of those titles made it the end of the 1990s, either). So while today, it might sound ludicrous to keep a title going through reprints, it made slightly more sense back then. Even then, however, most titles that went reprint-only quickly folded. X-Men, however, kept on going (bi-monthly, of course).
Therefore, if someone was going to reverse course on the X-Men's trajectory as characters, it was going to take one heck of a story. Luckily, Len Wein and Dave Cockrum had just the story in mind.
The X-Men were not non-existent in the early years of the 1970s. One of the team members, the Beast, even got his own short-lived feature in Amazing Adventures where he was transformed into a literal beast. Gerry Conway launched the feature, but it would soon be taken over by a young writer named Steve Englehart, as his first regular work for Marvel Comics.
The rest of the X-Men would make sporadic appearances in other titles, including a prominent role in the end of Englehart's (who quickly became a major writer at Marvel) famed "Secret Empire" storyline in Captain America in 1974...
In 1973/1974 (precisely when is a bit vague), then-Marvel Editor-in-Chief Roy Thomas, who had been a longtime X-Men writer, decided he wanted to bring the team back. Thomas came up with an idea where they would become an international team that would travel the globe, recruiting new mutants from around the world (and, of course, fighting bad guys along the way). He initially assigned the idea to writer Mike Friedrich and artist Dave Cockrum, who had just come over from DC where he was the Legion of Super-Heroes artist, so he clearly knew hot to draw a team book.