Not sure what you mean by "I took apart my hard drive casing and put it back together" but is sounds pretty clear that you did not either reassemble it correctly or that you may have damaged it during disassemble. Why would you mess with it in the first place? Please describe in detail what you did to the drive. What is the brand and model of the drive and how old is it? If you have physically damaged the electronics or the internal mechanics there is no practical way to fix it. I assume you have made sure that it is properly connected to the system (power and data). When you try to boot up, does the HDD light on the case flash? Have you booted to the bios and checked to see if the drive is recognized in the bios? If it is, is it selected as the boot drive? Try changing to boot drive order, make the HDD first and see what happens. Did you reconnect the data cable to the same MB header as before you did whatever you did?