The PeopleID column in tblAttendance has every row blank.
Delete the tblPeople_1 relationships and hide this table (it doesn't actually exist in the Nav Pane, it's an alias of tblPeople), then your form will work.
The (poorly named - because Name is an Access reserved word) Name field in tblAttendance is numeric and does work OK as a link back to tblPeople.
However, you cannot set referential integrity between tblPeople and tblAttendance until you clean up your data to remove existing 112 orphans in tblAttendance. Below query from the wizard shows the orphans.
Code:
SELECT tblAttendance.Name FROM tblAttendance LEFT JOIN tblPeople ON tblAttendance.[Name] = tblPeople.[ID] WHERE (((tblPeople.ID) Is Null));
I suspect that your "working perfect until today" database has been compromised by someone fooling around with the relationships, as several tables have 1,2 or even 3 aliases. Pretty hard to keep from screwing up the relationship links with that ambiguity.
A word of advice, you have around 50 tables and every one has a primary key named "ID". These keys have relationship links to other tables and those foreign key names do little to show the programmer (you and me) what table the link points to.
The working convention is to name all primary keys with a _PK suffix, like tblPeople_PK, then the link pointing back in tblAttendace be named tblPeople_FK (instead of Name). It makes life a lot easier.
The _PK is for Primary Key, and _FK is for Foreign Key.
And speaking of reserved words, Date is also an Access reserved word. In tblAttendance you should change that field name to something like AttendanceDate, which describes it exactly as a bonus.