You've really not given us much to go on in trying to help you. As John has suggested, Properties do get accidentally changed. Read/Write Properties of Folders, or for some users' access to Folders, are sometimes changed, often by IT/Network admins without end users being notified. Apps do get corrupted.
Has anything changed, design-wise, recently?
Is the Form based on a singleTable, or multiple Tables, i.e. on a Query?
If based on a Query, can New Records be added to the Query when opened independently?
Is this in a multi-user environment?
If #4 is true, is the app Split into a Front End/Back End configuration, as it must be, with each user having a copy of the Front End on their PC?