If you are unsure then it is best to advise not attempting any repair to be honest.
If the unit has not been moved recently then it is not likely to be the connections, however….
Try a mains power on/off first see if that fixes the problem, if not then:
1. Disconnect from mains power
2. Remove the lid – 3 screws on rear and one each side of unit.
3. HDD will be obvious, looking from front of unit it is located to the front left and has 2 connectors.