I have upgraded from v3.5 and now I am getting a blank page giving"Block doesn't exist:links_block" error message when I click a product link. I have turned on and off firenly url feature but could not solve the error. Is there any ideas?

Have you modified any templates, or is it a factory product (nothing modified) that you're using?

First check that the database is updated.
 
System > Database Management should give you a button if it hasnt been done. You do need to have permission set in the first section of the appropriate System > Privilige Groups

I have modified the templates. So if I use my own template I am getting the error. But If I choose one of the viart templates, it works properly. So I need to find which template file gives the errror? But I could not find it.
I have checked the "products_details.html" it seems there was no difference so which files should I look more?

I have updated the database. I think th error comes from the templa files that I modified. But I have modified almost all of them so I could not find which one causes the error message?

ViArt suggest that if you modify templates you should put them in a new folder - lets call it "templates/kempo" - and then in 3.6 use Administration > Layouts > Edit Layout to select your folder.
 
If the program cant find the modified template in your folder it looks in "templates/user/". So all you have to do is move the templates out of "kempo" and then move them back in until you find the rogue one.
 
Then you can use a difference tool to compare the two files to find the error.