Difficulty shooting in high ISO with my Canon 5D Mark II

While trying to shooting at 2500 ISO in a dark music jam, when I push the shutter it just stays open, the plus/minus exposure dial through the lens flips to far left, and I have to change the setting to make the shutter finish.. then no shot… do I have wrong settings somewhere that is causing this.? I have a 35mm 1:1.4 lens attached…Canon