Ora-02291 error when inserting data

[SvetlanaC]

ORA-02291: integrity constraint (ME_1944_SQL01_ S30.SYS_C002021 8404) violated - parent key not found
This is the answer I receive when I try to insert data into table. Two days ago there were not any problem, parent table was not changed but when I try to add data I receive this message, although parent key exists.

Please, help if you know what is this about.

Thank you

[Rabbit]

We need to see

A) the design of the parent table
B) the design of the foreign table
C) the values you're trying to insert
D) the values of the parent record