As I briefly talked about in Step 1, desktop computer cases come in many forms. If you need help closing your PC's case, please check your computer or case manual.
Note: If your computer was powering on properly before you reseated the internal cables but after the reseating it is not, follow the steps in this guide again. You likely forgot to properly plug back in a power cable or data cable.
Note: If you've reseated the internal power and data cables as part of a troubleshooting step, you should test to see if the reseating corrected the problem. If not, continue with whatever troubleshooting you were doing.