I took my dad's computer apart and cleaned out all the dust today and now the hard disk doesn't spin up anymore. It's an IDE hard disk and it was working fine before. When I boot up, the BIOS says: No disk found. And then says something like "Insert boot media and press enter" All the necessary cables are in.
I didn't take off the motherboard or take apart the hard drive. I just cleaned out the front cavity of the case, inside the PSU, case fan, CPU fan, and heat sink.
Update 1: Tried the HDD on my mom's computer. It works fine. Then I put it back in my dad's computer and during boot, it seems to spin up. I rebooted a few times with my ears right above the HDD and it sounds like it spins up for a few seconds and then just stops with a little noise
Update 2: Never mind. My mom's PSU is too old. It's missing the 4 pin cord that goes right beside the big 24 or so pin MB cord
Update 3: Now that I think about it. He did call me over a couple days ago to ask me about this weird noise the computer was making. He thought it was the front fan but I thought it was around the PSU but I wasn't sure 'cause there was the case fan, CPU fan and the PSU fan all in the same area. It didn't happen again for a week or so, so I didn't think much of it