Reasons for involuntary childlessness
Someone may be involuntarily childless if they are infertile or have an infertile partner. There are treatments such as IVF that may be used, and adoption is another option.
Homosexual couples can't currently have children that are genetically related to both parents, but surrogate pregnancy may be an option. In the future, it may be possible to convert somatic stem cells to gametes, which would allow for gay people to have children that are genetically related to both parents.
Adoption is also an option.
Being involuntarily single
Well, actually, single women can easily have their own genetic offspring if they want, by going to a sperm bank. The cost of a vial of sperm plus insemination is in the ballpark of $1,000. Single men might be able to find an egg donor and a gestational surrogate, but the cost of surrogate pregnancy is around one hundred times as much as sperm, $100,000 or more, which is generally prohibitive. This discrepancy adds a quite literal economic angle to the common biological adage that "sperm is cheap, eggs are expensive".
Adoption may be an option, although there are often many restrictions on who is eligible to adopt that vary based on the jurisdiction. It used to be illegal everywhere for single people to adopt children, but now it is legal in many places, including in the US. However, single men looking to adopt are often discriminated against.
Single parenthood is often associated with poor child life outcomes, although some argue that this effect is mainly correlational, since single parenthood is linked to other traits like poverty. Biodeterminists argue that environmental factors don't affect life outcomes much at all.
Eugenics advocates, social Darwinists, and anti-natalists would argue that people who are unable to attract a partner probably have some genetic, mental, or economic problems that would make them poor parents anyway.