Along with the two runes that follow it in the Elder Futhark (Nauthiz and Isa), Hagalaz is a rune that could rightfully be considered negative in scope. Its traditional meaning is hail, which is generally considered to be destructive. When Hagalaz comes up in a reading, it is usually indicative of disruptive forces that necessitate change, usually within the querent. Oftentimes those disruptive forces originate in the querent's past, involving lessons unlearned and issues unsolved that are affecting the present.
Sometimes, Hagalaz indicates events that come as a shock or surprise; surrounding runes will often indicate whether those events are positive or negative, but do not be surprised if negative events are most often indicated. Keep in mind, however, that sometimes things will appear very negative, but given time, a positive outcome may be realized. This is often the case with sweeping, unexpected change.
This rune is often associated with Urd, the Norn who is identified with the past.