Let me try to answer this question
Your code is:
RewriteEngine on
RewriteCond $1 !^(index.php|assets|images|js|css|uploads|favicon.png|install|sitemap.xml|robots.txt|update_database)
RewriteCond %(REQUEST_FILENAME) !-f
RewriteCond %(REQUEST_FILENAME) !-d
RewriteRule ^(.*)$ index.php?/$1 [L]
<Files 403.shtml>
order allow,deny allow from all </Files>
AddHandler send-as-is .asis
For a reason like on What is this file in .htaccess? you can remove the code as follow:
<Files 403.shtml>
order allow,deny allow from all </Files>
Then I think you can access your website again as I was tried it on my localhost and then I can accessed it too.
You can inform to you webhosting company to confirm the existence of code as follow on your .htaccess file because of some people think it's a XSS backdoor program (https://forums.digitalpoint.com/threads/files-403-shtml-order-allow-deny-allow-from-all.2748859/#post-19140446)
I hope this answer can help up you.