All of that is quite true, with most modern airliners almost all of the flight surfaces and controls are operated via a hydraulic system which in turn is powered though the engines (we can thank Howard Hughes for that) , so if the plane loses power most of the flight controls are rendered next to useless and then your left with an aircraft with the glide ratio of a ton of bricks. So the next thing is do modern airliners have a backup and/or emergency power supply so in case of lose of power the flight controls are still useable, or do the planes have secondary controls in case of emergency?

The answer is yes, of course they do. Accidents from the famous US Airways 1549 to Air Transat 236 show effective use of that backup power. The problem is that in an emergency where the plane goes violently out of control or suffers structural failure these do no good. The crew need to have time and chance to use them.

Naturally of course, with current technology you can't really duplicate the amount of power engines can produce with a backup system, because you would need something that can generate as much hydraulic pressure/electric power. Sure, you can have that, but that would add tons of weight to the aircraft and make it uneconomical in every way. The backup systems are usually up to the job when the plane is salveageable and the crew react properly to the emergency, and it's unlikely that fully powered controls can save a plane that has already suffered a violent loss of control or a structural failure.

As already said, yes they have back up power system. Most have auxiliary power unit (APU) and ram air turbine (RAT). What potential secondary system you had in mind? Direct control via control cables is impractical because (especially in case of large civil airliners) force required to move control surfaces (ailerons, elevators, rudder...) manually is too high.

As CCIP pointed out technology hasn't advanced far enough for a high output compact and light weight back-up power system. In a few years time who knows what we'll have, maybe a system that uses hydrogen and oxygen while flying at altitude? Frankly a back-up system that can reproduce the power of four or more jet engines might never be invented.

I do understand that direct control via cable is impractical on todays airliners which is exactly why I mentioned Howard Hughes because when he was building the H-4 Hercules otherwise known as the "Spruce Goose", Hughes found that the control surfaces where too large for the conventional cable linkages used at the time.

Also as has been pointed out that the crew needs time to respond to a situation and in a number of instances where a plane has crashed the crew either didn't have time to react or the damage was too great for the aircraft to continue to maintain airspeed and attitude or in this case a violent and possibly sudden weather system. In those cases nothing can really be done, and if something could be done it ends up being a split second reaction which can still go wrong.