I had this happen recently on a list of mine. The issue turned out that I turned off the tool bar in the web part settings. Once I turned it back on then you can access the form icon in the ribbon. This is very confusing because it turns out that the InfoPath icon still appears but the Nintex Forms and the Nintex Workflow icons disappear completely. So, just go back to edit web part and set the toolbar to “full toolbar” and then turn it off once you are done (or to whatever you want to change it to). The setting only appears to affect the icons in the ribbon.