The truth is, you can not really restore the form but if you do not know the fields used on the form, there is a way to find that out and re-create the form.
If you know the web-page, this form was embedded on, do following to find out the form fields:
Re-create the form with these fields and get it replaced on the web-page/marketo LP. Hope this will help.
Yes, Amit is correct. Marketo support won't be able to restore deleted forms, but then the only way is to manually recreate the form or to clone it if you remember the original form it got cloned from.
Marketo Support cannot restore deleted programs or campaigns as well. Marketo Support can only restore deleted leads, emails, and landing pages within a certain amount of time after deletion. Landing pages and emails that are assets of a deleted program cannot be restored.