User Profile Synchronization Service failed to start..can be fixed

if you want to do a User profile import in SharePoint 2010.. you should have the
'User Profile Synchronization Service' started. But there are many reason why this
service wont start. Following are some points to check before doing something crazy
like re-install the farm.

Pre-requisites

• The farm is running either the Standard or Enterprise version of SharePoint Server
  2010 and you have run the farm configuration wizard. Profile Synchronization does
  not work on a stand-alone installation for SharePoint Server 2010.
• If you are using Microsoft SQL Server 2008, Microsoft SQL Server 2008 with Service
  Pack 1 (SP1) with Cumulative Update 2 (CU2) (http://go.microsoft.com/fwlink/?LinkId=165962)
  is required.
• The WCF hotfix (KB976462)
  for Windows Server 2008 R2 is installed.
• An instance of the User Profile Service application exists and is started. For more
  information, see
  Create, edit, or delete a User Profile service application (SharePoint Server 2010).

If you have all of the above set right.. Check the following..

1.    Determine which service account will be used to User Profile Syncronization (UPS) ..Goto CA > Application Management > Manage Services on Server > User Profile Synchronization Service > Click on Start. you will see a default account name already filled in. Make a not of this account. Wait, don't be in a hurry..just check the account used. When you are finished with all the steps below..come back to this step and enter the credentials to start the service.

2.    Make sure this service account is a domain account with local administrator permissions
on WFE servers.. if not add the user account to the administrators group and Re-boot
system ( resetting IIS and Timer service may also work.. if not you have to re-boot)

3.    DirSync permissions set for the service account. To be sure give permission for
both "Replicating Directory Changes" and "Replication synchronization"

http://blogs.msdn.com/sharepoint/archive/2009/12/14/how-to-set-replication-directory-changes.aspx

4.    Service account has full control permissions on User Profile Service application. for this goto Central Admin >Application Management >Manage Service Applications >Click on the bar of "User Profile Service Application " (.. dont click on the link). In the ribbon, make sure that under Administrators and Permissions Menu, the service account has full control.

5.    Make sure both Forefront Identity Manager services are configured to run with the
service account identified. you can configure logon account by rightclick on service
> properties > log on .
Note: that the services may not be running. it will automatically
start when the User Profile Sync starts.

6.    Go back to step 1 and fill in the credentials and start service