Hydrochloric acid denatures amylase due to it's high pH. This pH change therefore permanently distorts amylase's conformational shape due to the breaking of ionic bonds in the enzyme's tertiary structure. Amylase's active site is consequently destroyed and enzyme-substrate complexes will fail to form.
Does this cover Hydrochloric acid's effect on amylase entirely?
Yes, but it's not only the ionic bonds that break in the protein's (enzyme's) tertiary structure. When an enzyme is denatured, all its bonds are broken; this includes the hydrophobic interactions, that hold a protein's tertiary structure, and the
covalent bonds that form between cysteine residues (disulphide bridges). You don't have to mention all of this, obviously, but just don't restrict your explanation to the breakage of ionic bonds only.
EDIT: Thanks again T-Rav!