Couldn't find answer online, so I'm turning to this community for an answer, Could Senator Sanders, after being taken off of NY's Democratic Primary ballot, legally 'Unsuspend'("Restart") his Presidential campaign after endorsing his opponent, Fmr. VP Biden for president? If not, then explain why not?
Yes he can, but not probably not as a Democratic candidate. Bernie is allowed to re-enter the race at anytime before the DNC scheduled for August, but doing so won't trigger a recount in for states where he wasn't on the ballot. Biden is about 400 votes ahead of Bernie (1305-935)in a race to 1991 votes, there are multiple scheduled primaries that will widen this lead now that there is only one candidate. So is it possible for Bernie to win the Democratic ticket? Yes. But almost impossible. The Democrats have practically selected their candidate.
He can run either as a third party candidate or an independant. Theodore Roosevelt did this when he was denied the Republican ballot in 1908, and while he didn't win, did get a significant amount of votes.
Intrestingly enough, Bernie doesn't even have to run. In 2016, despite dropping out, he was actually "written on" to the ballot by one of the Hawaian electors. Similar actions were taken by 6 others as well, although for different candidates (Source: NYT). This is only legal in a few states, so it is not really possible for him to win the electoral college, but it still could be possible for him to win the election, as follows: If he gets written on to ballots and niether Biden nor Trump win 270 electoral votes the vote goes to the house, as constitutionally, the house would take the top three candidates in terms of electoral vote. As the house is pretty progressive there's a chance he can win. But let's face it, however much I want it to happen, this is highly unlikely.
Until the convention, where the candidate is selected and affirmed by the party, any other candidate could 'un-suspend' their campaign and put themselves back in the running. Heck, someone could start a new campaign as late in the primary season as they like; there are no rules preventing it.
However, note that campaigns are only suspended when a candidate loses momentum to the point that investing further money seems like flushing it down the drain; they only endorse other candidates when it is clear they have no chance of winning themselves and want to focus their supporters on a reasonable alternative. It would take a significant groundswell to change those assessments — either a tremendous jump in polling or a tremendous influx of cash that indicates new support — and those eventualities are increasingly unlikely as the primary season drags on.
I imagine that under certain circumstances — say if the presumptive nominee died, or was arrested for (say) murder — then any number of candidates who had suspended their campaigns and endorsed the presumptive nominee would suddenly un-suspend and un-endorse in the hopes that such traumatic events would propel them into the lead. But it would take something on that level of significance to make un-suspending a campaign worthwhile.