Three days ago when I was leaving the store, I started up 2006 Chevy Trailblazer Ls and it wouldn't reverse. I tried to move forward and that wouldn't work either. When I switched from drive or reverse back to park, there was a loud clicking noise that occurred each time for a few seconds. A guy at the store who stated that he was a mechanic said that it was an issue with the transmission and suggested adding a quart of fluid, but that didn't solve the problem. Next, he suggested unhooking the negative battery cable and hooked it back up and the truck was able to move. I took it straight home which is about 2 minutes away from where I was at, but I noticed a reduction in power and a bumping noise when I accelerated. After getting the fluid level full, I was able to drive around with no issues. Today when I started up my truck, it went into reverse while parked and it felt like it was slipping again as I drove it. Have any of you experienced these symptoms, if so, what was the cause of it?