Yes, it's happened several times. In 2004, #25 Utah State was left out. UNLV at #25 was also left out in the 1990's.
In the 1970's, prior to conference tournaments, only one team from each conference made the tournament. So, in 1971 #2 USC did not make it because UCLA was #1 and beat USC twice that year. In 1973, #3 Maryland did not get in because they could not get by NC State.
16 seeded Harvard defeated 1 seed Stanford in the 1998 women's NCAA tournament. That is the only victory by a 16 seed in either the men's or women's tournament.
In NCAA basketball history, there have been a few instances where teams ranked 15th defeated teams ranked 2nd in the NCAA Tournament. Notably, this occurred during the 2012 tournament when Lehigh defeated Duke, and again in 2016 when Middle Tennessee State upset Michigan State. These upsets are rare, highlighting the unpredictability of the tournament. Overall, there are only a handful of such occurrences in NCAA tournament history.
1998
Rankings are subjective. The NCAA Tournament games are about matchups.
That was Villanova, ranked 8th in the Southeast bracket, when they defeated Georgetown, 66-64, to win the NCAA championship in 1985.
Villanova was the lowest seed to win the tournament as an 8 seed in 1985.
No, Illinois has never won the NCAA Men's Basketball Tournament.
The lowest seed to ever win a NCAA Basketball title was Villanova, ranked seed #8. They won the tournament in 1985. Lower seeds have made it to the Final Four (George Mason and Louisiana State, ranked 11th, made it in 2006 and 1986 respectively) but never won. UCLA in 1980 was ranked 48th and made it to the championship game.
No they have not.
They missed the tournament in 2010. Prior to that they missed in 2003.
NO
Since the seeding system was introduced in 1979, the lowest seed to win the NCAA men's basketball tournament was 8th-seeded Villanova in 1985.