Hi Muhammad Shoaib,
I'm sorry to hear about your experience. You can report a client either by clicking on the cog wheel next to their messages and reporting it as inappropriate or by flagging their job posting. Here's the help article for more information. The team reviews all reports against Upwork ToS and takes actions as needed.
Start working on new tasks once a previous Milestone had been released and a new Milestone has been created and funded. Please find more information in this Help article.