Let's say that either the Democratic or Republican party, at some point in the future, stops existing in its current form (for whatever reason). Has there been, in American history, anything that would be similar to that? Where a political party of that size just imploded? What happened? Were there other parties vying to fill the political vacuum? How did one of the parties eventually end up filling that vacuum? How long did that void last? If the party imploded into multiple factions did one of the factions end up filling the vacuum, or was it some third party that was previously on the fringes?
Note: I'm not trying to get anybody talking about modern politics, I just used Democrat and Republican because I'm wondering if a party of that scale (where there were basically no feasible alternatives) has ever collapsed.
Yes. The Whig party, and to a lesser extent the Democratic Party both collapsed in the 1850s.
The Whigs in the 1830s through the 1850s promoted central banking, education, and infrastructure. It was basically a pro-industrialization party to oppose the pro-agrarian Democratic Party. They were able to elect a couple of presidents and formed a solid second party, with the Dems usually coming out on top in most races for Federal offices. This is generally known as the Second Party system.
By the 1850s slavery was becoming an increasingly important position. And neither the Whigs nor the Democrats had a single solid position. Both had pro and anti-slavery wings. As that began to become THE issue in American politics the two major parties tore themselves to shreds over the issue and broke into multiple minor parties: Republican, Free-Soil, American (Know-Nothing), Opposition, Constitutional-Union, multiple branches each claiming to be the "true" Democratic Party and more.
In the end this devolved into the election of 1860 where four candidates received electoral votes: Lincoln - Republican, Breckenridge - Southern Democrat, Bell - Constitutional Union, and Douglas - Democrat. Lincoln won with only 39.8% of the popular vote. The Civil War commenced shortly into Lincoln's first term. This allowed the minority Republicans to become the majority party in the North. They largely adopted Whig policies, with a blending of some Free-soil and Know Nothing ideas. So even as the war raged, the Republicans were pushing for infrastructure projects (rail to the Pacific), education (like land grant universities) and centralized banking. They also pushed a lot of progressive ideas like women's suffrage and black rights. The Democrats reorganized (having never completely disappeared) as the pro-agrarian conservative party. This became the Third Party system. Since then the two major parties have radically reorganized, but neither has completely dissolved like the Whigs did.
You may notice that I skipped the First Party system. It was a kind of wild and woolly system that doesn't look much like the party systems we are used to today. There were parties Federalists, Democratic-Republicans, National Republicans, and Democrats. But party affiliation was weak, and frequently driven by individual personality. The election of 1824 saw four separate DR candidates get electoral votes. And when the sitting president John Quincy Adams had problems with the DRs he had no problem splitting off. But if you wanted to include the Federalists and National Republicans as failed Whig like parties, that would be reasonable.