Cut copper traces are not easily repairable using things around the house. Traces around the CPU are typically critical to the motherboards functionality.
First a few questions:
- Is the cut trace visible? Typically copper traces around the processor are 4-5 mils wide (1 mil = 1 thousandth of an inch).
- Is it just one trace that is cut? If a improperly installed heatsink has cut a trace, then more than just one trace could have been damaged.
- Can you check with a meter to make sure that the trace is disconnected?
- Does the cut trace connect to the memory slots or chipsets?
I've intentionally cut traces with a scalpel and it takes a little effort to do correctly. A heatsink cutting a trace is very unlucky.
To repair a trace (depending upon the size of the gap), I would use some solder and a small piece of bare wire, if necessary.