After your trade deadline passes, you can longer propose or accept trades.
For example, if your deadline is Week 10 like the image above, after your league's records are updated for Week 10, typically a few hours after Monday Night Football for Week 10 ends, you will no longer be able to make trades.
It is possible for trades to be accepted prior to your trade deadline, and then processed afterward. That can happen if you have a review period is enabled:
When can our league trade again?
For dynasty and keeper leagues, trading will reopen as soon as your league's playoff have ended.