It could be possible that there is not enough hydraulic fluid in the tank to allow it to go up and it pushes air into the cylinder which will compress and lower the platform. I would send the platform to that height using the ground controls and check the level in the tank to make sure it isn't empty.
The hydraulic motor/pump will also make a different noise when it is not pumping a fluid.

I also forgot to answer your other question below:
If someone did lift over the maximum capacity I would expect the issue to happen the entire time it is lifted. They would have had to put a lot of weight on it to cause permanent damage.
If I remember correctly the weight limit is 350lbs for the operator platform and 200lbs for the shelf. The shelf would break before any damage would be done to the hydraulics.