The friction disc either needs tightened up, or has fallen apart. If you look where the three point lever pivots, there should be a fiber disc between the lever and the housing. If the disc is missing, the lever needs to come off to replace the disc. If the disc is still there, then you just need to tighten up the nut on the end of the shaft that the lever is mounted on. There is a spring behind the nut that could have broke as well. The spring keeps the pressure on the disc. The lever could be seized on the shaft, so watch  about tightening the nut too much, you could strip the threads off the shaft.