The default View on many modules will be set to Recent Items. This view is set to show records which have been created/ edited recently - for many modules this is the last 21 days, for other modules this is set to the last 7 days. So if you have added a Contact to your system, 22 days later this person will disappear from the Recent Items view.
You have a few options to get back to these records.
Use a different View. Select a different view, for example "All Contacts". (You need to be sure that this view contains data which will be visible to you!)
Records are displayed in OpenCRM according to which View you are using. A View is essentially a pre-saved search, allowing you to find specific groups of records quickly and easily. This FAQ looks at how you can create Custom Views: https://opencrm.co.uk/help/?kb=2997890&article=What-are-Custom-Views-and-how-can-I-set-these-up
Your system admin can change the default View, so each time you go into the module in question you do not have to switch between views. This is set on the "Access Information" panel of the View edit screen, by using the multi-select picklist to choose the users, or all users on a particular Profile you want to set this view to default for:
Your system admin can change/ extend the number of days showing on the Recent Items. This can be done by going to Settings, selecting Additional Settings and editing the relevant Recent Item Settings.