What to do in such a case - I was a guarantor for the borrower...?

According to Clause 15 of the Transitional Provisions of the Land Code of Ukraine, until January 1, 2024, it is prohibited to buy and sell or alienate in any other way in favor of legal entities of land plots that are privately owned and classified as lands for commercial agricultural production, land plots allocated in kind (in the area) to owners of land plots (shares) for personal farming, as well as land plots (shares), except for the transfer of land ownership rights to banks as a pledge, transfer of land plots as inheritance, exchange (mines), respectively to the second part of Article 37-1 of this Code of a land plot to another land plot with the same normative monetary value or the difference between the normative monetary values of which is no more than 10 percent and alienation of land plots for public needs.

Agreements (including powers of attorney) entered into during the period of the ban on the purchase and sale or alienation of land plots and land shares (shares) in another way established by this subsection, in terms of their purchase and sale and alienation in another way on in favor of legal entities, as well as in terms of the transfer of rights to alienate these land plots and land shares (shares) in favor of legal entities for the future (including the conclusion of previous contracts), are invalid from the moment of their conclusion (certificate) .

Therefore, according to the above, the bank (independently or through collectors) cannot take away your land share.