What does it do when you try to lower it? Does it try and fail, or just ignore you?
If the former, then perhaps there is something blocking it from coming down, or something jamming the mechanism. If the latter, it is probably not getting power. Check any fuses first, and then start tracing the circuit with a voltage meter.
Need a little more info such as do you hear a hum, click or anything? As John said there could be fuses or some safety device if everything is not in order before lowering. Most likely a fuse, switch, wire or motor. Get out the VM and start checking and let us know.