I am customizing the newform.aspx, and now when I try to add another text-box SharePoint field and try to change the type to form field, the form field option don't appear in the drop down box. I think I accidentally overridden or deleted a few tags. How can I restore the newform.aspx to its earlier version?
This link has step-by-step instructions to restore your newform.aspx. Open the broken form and another working form in SP Designer, remove the ListFormWebPart from the page, then copy the ListFormWebPart from the unbroken page. You also have to update the List GUIDs to make sure the web part is pointing to the correct list.
As an aside, in future you could create a new form (or copy newform.aspx) and then use SP designer to point the list to the new form. This would allow you to recover more easily.