List Spontaneously Stopped Creating New Items

I have three different lists with forms customized through PowerApps which have all worked perfectly up until yesterday when they suddenly stopped creating new items. The user fills out the form and clicks Save and nothing happens--the form remains on the page, no new item appears in the list. I haven't made any edits via PowerApps since last year, and some 1500 items have been successfully submitted on all three lists. They just spontaneously broke. Anybody encounter this before? This is the modern SharePoint Online/Office 365 environment. 

Thanks in advance!