combobox lose selected valeu and bound items after page postback

Hi,

I create a visual web part in SharePoint 2013 and use the combobox control in the web part . I bind the combobox when page load (if (!this.Page.IsPostBack)), the issue is when I select a value in the combobox and click a button(check) to filter a gird, it works well, but after a while such as 20 minuets later, when I click the button(check) again, and the page postback, but the combobox is blank and lose bound items and selected value .

Can any body tell me how can I retain the selected value and the bound items after a postback ? By the way, it happens I open the page and leave it some times later(20-30 minuets).

Thank you!