Drop down error message

Good morning,

I'm hoping someone can help me with this real quick - I have a field that is a drop down and the selections for that drop down are from another table.  It's been working fine until today, and now I am getting this error message:

Does anyone know what this means?  And how do I fix it?  I checked the properties for both fields, and as far as I can tell nothing changed, but I must be missing something.

Thanks in advance.