L.M.
Being in the office, you may have been able to notice if business has picked up. Last June my hours were also reduced, and I know things are still bad. Two people retired and they weren't replaced. Just make sure this isn't the case in your office.
A good way to approach the subject is by saying something like, "It looks like business/work load has been increasing lately, if you would like me to put in some additional hours each week, I would be available".