The latest episode of The Flash reveals that the Speed Force is dead, and confirms that Barry Allen (Grant Gustin) is directly responsible for its death. While on Lian Yu in the midseason premiere, Barry noted a problem with his speedster powers, but wasn't able to identify the cause. As it turns out, it was a sign that the Speed Force was dying.
In The Flash season 6, episode 14, titled "The Death of the Speed Force", Wally West (Keiynan Lonsdale) returns to Central City with a whole new level of control over his powers. Wally explains to Barry that he noticed a disturbance in the Speed Force and accuses Barry of being responsible once he learns about Barry's battle with Bloodwork (Sendil Ramamurthy) in the first half of The Flash season 6. Barry had a confrontation with the Speed Force after being infected with Bloodwork's energy. It appeared that this was what weakened the Speed Force, but after talking with it directly, Barry is able to discover the real cause.
The Speed Force is now dead, and what killed it was Barry, even though it doesn't blame him. According to the Speed Force, it happened during the Crisis. Barry getting power from the Spectre is what set it all in motion. In "Crisis on Infinite Earths" Part 4, Barry and all the Paragons were trapped in the Vanishing Point when Oliver Queen (Stephen Amell) returned as the mystical entity known as the Spectre. At the time, Barry couldn't get into the Speed Force, so he asked Oliver for help. Barry convinced Oliver to help him use the Speed Force to save everyone. Eventually, Oliver agreed, but warned Barry that such a decision would be dangerous.
Oliver touched Barry's head and used his power to help Barry gain access to the Speed Force again. After this, Barry was able to bring everyone into the Speed Force. He of course had no idea that his actions would have long-lasting consequences. Apparently, the energy boost Barry received from Oliver is the catalyst that led to the death of the Speed Force. The Spectre's energy got into the Speed Force and tainted it until it slowly died.
In a way, the Speed Force has been Barry's most important ally on The Flash. Often treated as a sentient being, the Speed Force has guided Barry through many of his greatest trials - and now it's seemingly gone for good. Barry now has a plan in place to create an artificial Speed Force so that he can maintain his abilities, but it won't be just like the real one, which is an incredibly powerful source of energy that has fueled speedsters across multiple time periods. Unfortunately, the Speed Force has become the most unexpected casualty of "Crisis on Infinite Earths".