Problem

Your content database has corrupted and you are not able to access some pages / subsites / documents.

Solution

Try the following to see if it fixes your problem:

  1. Run SharePoint Management Shell
  2. Type Get-SpDatabase | fl name,id
  3. Locate the offending database and copy its ID
  4. Enter the following PowerShell commands to fix your content database.
$db = Get-SPDatabase (Content Database ID)
$db.Repair($false)
$db.Update()