Frozen as like filled with water then froze or frozen with rust?
Now that I think about it, it really doesn't matter, pull it out and put a new one in, as any fix is liable to only be temporary, and give you problems down the line.
There are tools that help force lube into the cable outer sleeve. If it is full of frozen water then this would not help until you thaw it and then force the water out.
See this link for an example - they are usually locally available at motorcycle shops