Dude, that same thing totally happened to a computer I worked on, except that instead of a power surge to heat things up and melt the solder, it was smothered in a small desk and eventually got hot enough to melt it. Basically, I just fished around and found the other hook hiding near another component.
I really don't know what you should do if you can't find the other hook. I don't know if you can buy them, as I'm not even sure what it would be called. From the pictures, it doesn't look like it has the standard holes for another heatsink to be added. This is a toughie. Maybe take the motherboard out and get a small piece of wire and string through the holes, then try and solder them into place. Very carefully, might I add.
This message thread is one of many I found when searching the 'net. I've replied to a couple others already.
Last week my dad called about the computer not showing anything on the monitor. I thought the power supply had failed until I opened it up and saw the heatsink laying in there unattached. One of the two clips was missing (upper right as computer sits in normal position).
Found the clip after shaking the computer upside down, put back in with "super glue" on it. Bent the wire spring of the heatsink straighter so it had less strain on the clips. Thermal paste was still sticky so I left that as-is, not having any available ATM.
This was a HP Pavilion a810n and other than one other identical model mentioned doing the same thing these others being talked about are apparently different motherboards. Only similarity is how the heatsink attaches. Seems like a manufacturing defect or bad design if the solder heats up enough to let loose, or maybe just weak soldering in the first place.
I've used notebook computers instead of a desktop PC for over 7 years but I also have a DELL 4600 and that was one said to have a heatsink problem so guess I better open it up and look inside.
Everyone using a desktop PC probably should check about this. Although, I've seen people say the computer can tell if the heatsink loses contact (either thermally or electrically thru those clips?).
Glad to report this one of my parents was fixed and working okay after doing a disconnect/reconnect of the PSU to MB cable while plugged into the wall. But once its off AC and returned again the problem happens again where the fix needs to be redone to restore internal power.
Maybe this varies PC to PC when the heatsink causes trouble.