Hi I recently was confirmed for a job, and completed the work. The client then said that I hadn't accepted the proposal so he couldn't pay me. Now they are unresponsive to messages. Please help.
You must never start work until a formal Upwork contract is in place for either a fixed-rate job with a milestone and the agreed amount funded into escrow, or as an hourly job with the tracker turned on. Otherwise, Upwork can't protect you from scammers.
All you can do now is to report the client to CS and move on. If it was a writing job, and the client publishes it on the Internet, then you can ask for a DMCA takedown.
You should familiarize yourself with the site to avoid this happening again. Have a look here and particularly read "Tips to avoid questionable jobs":
If you did not accept the offer, there was no contract and the client indeed could not pay you. Is the offer still active? If it was less than 7 days ago you might still be able to accept it....
The client has done nothing wrong, they sent an offer. If you chose not to accept it, that is hardly the client's fault and there is nothing to report the client for at all.
If the client is US based remember it was the 4rth of July weekend, and a lot of people have been off work since last Friday.
It looks like the offer your client sent has unfortunately expired. I see you asked them to resend it, so please keep following up on your conversation and keep communicating with your client. Going forward, make sure to accept the offer before doing any work on a job.
Please review the freelancer resources Nichola shared, especially the Safety First section, and let us know if you have any questions.