When a Proposals expires its status automatically changes to "archived." The expiration date set by the user in Create/Edit > Proposals > Compose - Step 1 determines when this will occur. However, you can change an expiration date at any time.

Select All from the Currently Viewing drop-down list located on the Activity page to display all your Proposals and Cards, including archived items. Or, use the search tool on the Activity page to locate a specific Proposals.