Would frozen holy water still retain its blessing? Say that a person had a container with holy water and the water within it froze. Would that frozen holy water still retain its blessing and be efficacious in protecting against evil, be able to be used in baptism, etc?
Changes of state are accidental changes, not substantial changes—viz., frozen water is still water, despite it changing temperature, density, having a different amount of air dissolved in it, etc.

The 1957 Missal's De Defectibus ("Defects in the Celebration of the Mass") contains this statement about if the consecrated Blood of Christ freezes:


  
  Si in hieme Sanguis congeletur in Calice, involvatur Calix pannis calefactis: si id non proficeret, ponatur in ferventi aqua prope Altare, dummodo in Calicem non intret, donec liquefiat.If in the winter the Blood freezes in the Chalice, wrap the Chalice with warm cloths: if this does not work, put [the Chalice] in hot water near the Altar, such that the it does not enter the Chalice, until [the Blood] liquefies.
  


It does not say that frozen Blood of Christ ceases to be His Blood. Similarly, frozen holy water is still holy water.