This issue isn't actually related to the restoration. It has to do with your server setup. I understand that you don't have anybody else to help you so I'll try to do so despite this issue not really being within the kind of issues I am supposed to offer support for :)
The error message tells me that there is an error with your "student" template. Seeing its name, I assume it's a custom template, probably made with Artisteer, so there is no point in telling you to ask the template's developer for support. In this case let's take a look at the rest of the message. It tells you that the function mb_strpos() doesn't exist. This means that your host has not enabled the PHP extension mbstring which is actually required for Joomla! to function properly. Please tell your host to enable the mbstring extension and your site will work properly.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!