GA pilot reported loss of aircraft control due to extreme turbulence encountered while flying through a cloud bank. Upon exiting the clouds; pilot regained control and continued the flight.

Narrative

I was cruising at 8;000 feet in a Bonanza on an IFR flight from ZZZ to ZZZ1 and approximately 30 NM's North ZZZ2 when I entered a benign cloud bank but shortly after going IMC; I encountered localized extreme turbulence; hitting my head on the ceiling; and the autopilot disconnected. I was stunned momentarily but then realized the plane was in an unusual attitude (i.e. dive) and with very strong downdrafts and heavy controls. I recovered from the unusual attitude and I informed ATC that I had encountered extreme turbulence & downdrafts and could not hold altitude. Exiting IMC; the turbulence subsided and I had lost several thousand feet of altitude. I requested and was cleared to 6;000 and completed the flight without further incident. I think that the extremely high temperatures (near 100 degrees) on the ground and flying in this afternoon heat cumulatively contributed to a localized turbulent weather condition. The weather picture on the avionics did not show either precipitation; lightning; nor storm cell attributes at my position (or projected path); however there was evidently enough energy in the air to cause the extreme localized turbulence and downdrafts encountered.