Ashes are gravid with a multiplicity of aligned meanings. This is apparent when the Shulchan Aruch mentions that a chosson should have ashes placed on his head as a sign of mourning over the loss of the Beis HaMikdash.
Mourning over this massive loss of our physical conduit for the Shechinah’s presence on earth carries with it the divine impetus to effect change, to act in a way that merits the re-establishment of the Beis HaMikdash. For every day it’s not here, Chazal inform us that as Klal Yisrael we haven’t yet acted in a way to create the conditions for its re-establishment. It’s a mourning for what we no longer merit but it’s also a promise for its return when we act in accordance with our higher purpose as Jews, as Klal Yisrael.
The impetus of ashes requires us to align our free will with the expressed will of Ribbono Shel Olam. Ashes are a sign of our sorrow, but also a sign of perpetual inspiration to do teshuvah, to do mitzvos in a way that we are promised will lead to a permanent end of all our sorrows.