Standard procedure,
Step 1:
Take out battery, insert it in again (remember to switch off your camera before doing step 1), switch on camera and try again.
Step 2:
If step 1 don't work, remove your lens, and reinsert again, might be because you did not insert properly, then try again.
Step 3:
If Step 2 don't work, remove the lens, and change another lens to try.
Step 4:
Still cannot... bring down to CSC to check.