I ran into a similar situation with some acquaintances - it's definitely scary, but the document hasn't gone anywhere. Contact the notarial chamber of the Republic of Tatarstan, where the archives of deceased notaries are kept. Call the chamber right away and clarify whether Ivanov's archive has been transferred, because the process can take a while. Ask them to help you locate your specific will using your grandmother's last name and the date it was drawn up - they'll find it faster that way.
Once the will is located, you'll be given a copy or granted access to the original. After that, you can approach any notary to get a certificate of inheritance. Just keep in mind that archives can be in transition for a couple of months, so don't expect lightning-fast results - but this is a normal, established process.