could be any number of reasons.
might be helpful to attach you file(s) to give others a chance to see what you're dealing with or offer a little more explanation.
my guess is you have features defined in the context of the deleted asm, or the parts got moved/renamed and now it is coming up with a non-matching internal ID.
otherwise try searching
"external references", "parent-child relationships"
Check out the external references...
You can 'allow multiple contexts for parts when editing in assembly' but being a new user and not fully understanding what the consequences of it means it probably isn't something you should turn on.