Justicia / Jacobinia carnea Problem

My little plant which started blooming suddenly "collapsed" yesterday. Had it in full sun but moved it to shade earlier in the week. What should I do ... have moved it back to sun. Sorry, they are a bit fuzzy! Thank you in advance.

 

Is the lower stem black?

 

Thanks for responding pete. Yes, the lower stem looks black so I have scraped it ... it was soft coming off but green within. Some of the leaves are crispy. It may have been over watered with the recent rains. I will keep it in the sunny spot to dry out and keep fingers crossed.

 

Not a plant I know anything about, but it does appear to have died upwards if you get my meaning.
Root rot perhaps?