What is the hard lump 6 months sfter banging lower leg muscle on corner of stairs?

Although it could represent a hematoma from bleeding due to the trauma, it's best to see one's doctor for an exam and possibly a doppler test, an X-ray or an MRI.

I agree that this is most likely a blood clot which formed from bleeding into that muscle. The fact that it's hard means that it didn't completely dissolve and has become fibrotic & won't go away. If it's a source of pain or tenderness, a surgeon can remove it via a small incision.