Can you get back a discarded change set?
Accepted answer
Assuming that you know a file name associated with the changeset, one way to get it back would be to first show history of the file and then using the View Menu in the history view, click on "Show all in Repository". This will show all the changesets. You can then pick your changeset and accept the changes into your workspace.
if you had it associated with a workitem, then open the workitem in Eclipse client, go to links tab, select the changeset and accept into your workspace.
Comments
In addition to the 2 correct approaches Surya mentioned, you can also use the "Search" --> "Jazz Source Control" --> "Change Sets" dialog. You can then fill in fields which might help you find the change set such as the component, or creator. Once the change set is found in the "Search" view, there will be an Accept action available on it.