Solution 1.
Take the lens off and try to power it on also,
Take the battery out and hold the power button and shutter for 20 seconds then proceed to insert the battery again. Does it work?
Solution 2.
take out the battery and if you can power on the camera without a battery
do so, it may force the camera to make a change in its memory and may show a different error msg.
Solution 3.
Make sure there is no Dust or Debris in or near the sensor, -or Mirror- (if its a SLR)
Solution 4.
There may be a Hardware issue preventing the Camera from Recognizing Lenses, You may have to contact the Manufacturer.
Solution 5.
GENTLY give the camera a good few taps on a table wrapped in a cloth and keep rotating the camera 360 degrees each tap, there could be a mechanical part that is stuck, it may become free.
High end cameras have built in software to prevent damage upon being dusty or mechanical parts being stuck to prevent damage to motors, gears and pins.
Has the camera ever been wet, Damp or suffered extreme heat?
Hope this helps. good luck!