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:

• Create a smart list to find all the leads who filled out a form on that specific web-page. Use From Name is any since you will not be able to  select the form name now. Add Web Page constraint and put the page URL form was embedded on:
  
• Go to People Tab to see all the leads who filled out the form:
  
• Double click on any of the lead to go to Lead Details page:
  
• Go to "Activity Log" of the lead detail page and filter the activities by web and find out the form which was deleted:
  
  
  
• Double click on the "Filled Out Form" activity within Activity log and you will find all the form fields.

Re-create the form with these fields and get it replaced on the web-page/marketo LP. Hope this will help.