I could not make it to church on Ash Wednesday. Is it a sin not to receive blessed ashes on Ash Wednesday?
No. Ash Wednesday is not a holy day of obligation, so Catholics are not bound to attend the Ash Wednesday Mass under pain of sin. It is good for Catholics to go, if at all possible, but not obligatory.
As for the blessed ashes, they are a sacramental and it is not required to receive them. Not only is Ash Wednesday not a holy day of obligation, should you choose to do so you could attend an Ash Wednesday Mass and choose not to receive the blessed ashes.