BS"D 27 Adar 5780
It says in Yevomos (63a), Trouble comes to the world only because of Israel. Now, in the past year that was a lot of bittul Torah, among our many sins. Therefore every one should strengthen himself with all his might and strive to increase Torah learning. It is in its [Torah] power to protect and save from all bad things as it says in Sotah (21a).
They also advised in times like this to fast on erev Rosh Chodesh Nisan for half the day and to daven the Yom Kippur Kotton prayer. The core of a fast is prayer. May HaKodosh Boruch Hu listen to our supplication and say "Enough!" to our troubles, and send Refuah Shleimoh speedily to all the sick of your People Israel.
Someone who wants to fast the entire day or half a day should remember to undertake the fast on Tuesday hbal"t in the Shemoneh Esrei of Minchah and then he will be able to say Aneinu in Minchah on Wednesday. Bedi'eved one may undertake the fast until the night, and some permit doing it even at night.
One who fasts for half a day without undertaking it in advance may not say Aneinu, but there is still merit in doing this.
One may say, when undertaking it, "Bli Neder" (without a vow), or "As long as I want to fast," [however if he does not fast until after Minchah he should not say Aneinu].
And the Merciful Father should say to the Angel of Destruction, "Enough!" soon.
(signed) Chaim Kanievsky