Access denied in SharePoint 2010

Some of the users are getting following access denied error upon looking in sharepoint logs. These users were earlier able to access the SharePoint site. But all of sudden they are getting "An Unexpected error has occured".

PermissionMask check failed. asking for 0x00000005, have 0x400000300C231061  
PermissionMask check failed. asking for 0x00000015, have 0x400000300C231061  
PermissionMask check failed. asking for 0x00040000, have 0x400000B00C631061  
PermissionMask check failed. asking for 0x00040000, have 0x400000B00C631061  
PermissionMask check failed. asking for 0x20000000, have 0x400000B00C631061  
PermissionMask check failed. asking for 0x30000000, have 0x400000B00C631061  

Nothing has changed on the server, and they have same permission as they did before.