I have noticed Musing voting power has been drooped so much. Is it because of HF20 issue or some major power down has happened ?

The voting Power of everyone was going down to 0% because of HF20.

It will take 5 days untill it is back to 100% (if you don't vote for 5 days)


Yes, it is because of the hardfork.  Everyone's vote is lower right now because their VP has been depleted.  You will start to see a steady increase in vote amounts as their voting power begins to regenerate.  

The rate for regeneration is at 20% per day so if they started at 0% and didn't vote on any posts their power would be back up to 100% after five days.  I have heard that a patch has been applied to increase the regeneration rate for a short time as a fix for everyone's VP being knocked down to 0. 

Give it some time and eventually the upvotes from musing.io will be back where you expect them to be.  As well as the upvotes of everyone else.