Ex-Broncos QB Chad Kelly pled NOT GUILTY to felony criminal trespassing stemming from allegations he broke into a Colorado home after Von Miller's Halloween party ... only to be chased out by a person beating him with a vacuum cleaner tube.

The 24-year-old appeared in court in Arapahoe County, Colorado on Monday to enter the plea. This is a huge deal for Kelly -- because if convicted, he faces up to 3 years in prison and a $100,000 fine.

The trouble for Kelly started after he left Von Miller's Halloween party on October 22 ... and allegedly wandered into a nearby home.

Cops say Kelly broke into the home around 1:17 AM and sat down on the couch next to a woman who was holding her baby. Kelly was allegedly mumbling incoherently at the time.

Police say someone in the home chased Kelly out while striking him with a tube from a vacuum cleaner. He was ultimately arrested for felony criminal trespassing.

Broncos GM John Elway was clearly pissed about the whole ordeal ... and cut the 24-year-old QB just hours after the arrest.

Elway and the team said in a statement, "Even though Chad's no longer part of our team, we've offered to help him however we can and are supportive of him in every way possible."