Your content database has corrupted and you are not able to access some pages / subsites / documents.
Try the following to see if it fixes your problem:
- Run SharePoint Management Shell
- Type Get-SpDatabase | fl name,id
- Locate the offending database and copy its ID
- Enter the following PowerShell commands to fix your content database.
$db = Get-SPDatabase (Content Database ID) $db.Repair($false) $db.Update()