Strange Issue..[Very Very Urgent]..

Dear All,
Since last few days , some of .asp file is created in our web folder at web server and even some folders also created.

Mostly all .html and .asp files only and when i open the file, it looks like some shopping web page and not sure how does comes inside our web folder and we are running out of idea. we are using instagram.min,js file as a third party file and rest is all our own code/

we have no idea how did the files in our web folder at web server. we have other web folder for other sites which is not affected , and only particular web site folder only got this.

Please give some advice on this.

Looking forward your valuable reply!!

Regards,
Mk