My Bloody Valentine had severe problems with the MPAA (Motion Picture Association of America) before the films release, they requested cuts to every death in the movie in order to attain an R rating; and even then the movie was given an X.
Reasons have varied over the severe cutting, the major one being the backlash of the violence in Friday the 13th a year prior. Another reason was the murder of John Lennon in December 1980 which caused major negative reaction to violence in media in the wake of his death.
For a long time it was believed that the uncut version of the film was the Holy Grail for Horror fans. Paramount refused to release an uncut version, to which Lionsgate acquired the rights for an uncut DVD release in 2009. George Mihalka stated that 9 minutes of cuts were made before release which caused concern for fans as when the new version was finally released only 2 and a half minutes were restored contradicting his earlier statement. He later explained that the other 6 minutes was expository footage that didn't impact the rest of the film and gave his seal of approval stating that this was how the film was meant to be seen. The original negative has either deteriorated beyond repair or has long since disappeared. The footage sourced was from the film's Producer John Dunning's storage facility in Canada; this print was 35mm and the footage that was restored was noticeably grainy and in low quality which required color correction.
A scene where a character is drowned by forcing his head in a boiling pot is missing a frame, possibly because of damage or loss. One scene in particular and possibly the most violent is where a couple gets impaled with a mining drill. Mihalka remembered filming this scene and is quoted as saying:
"There was a scene with the two kids in the mine and they’re joined together forever,” ..... “There was a whole set up scene to that. They’re just necking and Tom’s lying on top of [Harriet] and the miner comes in and puts the [drill bit] through him. She’s got her eyes closed and thinks he’s just being a little frisky. She opens her mouth and her eyes just widen as he bleeds into her mouth. Then the miner goes whoomp! And [skewers] them both. That scene had to be taken out totally. It has disappeared and we’ll never see it.” 
The film's original full running time was 99 minutes, the 2009 release is the salvaged 93 minute cut.
Movie Censorship's comparison between the R rated cut and the 2009 release