When I am trying to access other computer via the following ways.

Start > Run (Windows Key + R)

• \\ServerName
• \\ServerName\ShareName
• \\ComputerName\ShareName
• \\192.168.123.123\ShareName 

It’s giving the following error

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Windows Explorer
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\\server is not accessible. You might not have permission to use this network resource. Contact the administrator of this server to find out if you have access permissions.

Logon failure: user account restriction.  Possible reasons are blank passwords not allowed, logon hour restrictions, or a policy restriction has been enforced.
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OK  
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Solution :

• Start > Run > type regedit.exe and press enter 
• Then go to HKLM\System\CurrentControlSet\Control\Lsa\limitblankpassworduse
• change from 1 to 0

Note : Do not use search in windows registry - it will have many entries. Its faster if you go to the required key manually.