As mentioned previously, the Shulhan Aruch (O.H. 90:9) says that if one cannot pray at the synagogue, one should try and pray at the time that the congregation typically prays. The Mishna Berura (M.B., O.H. 90:32) says that one can choose a time as long as there is one congregation in one’s city that prays at that time. Evidently, in large cities, one would have many options of when to pray alone since there is sure to be a Minyan somewhere.
As to what justifies praying alone, the Mishna Berura (ibid:29) says that if one is ill, or even if one is tired but is not ill, one is exempt from praying with a Minyan. As well, if one will sustain a financial loss by attending a Minyan, such as leaving a potential client in order to attend a Minyan, one may pray alone. Additionally, the Mishna Berura (110:12) says that if one’s employer does not permit one to leave work to pray with a Minyan, one may pray alone. Missing Minyan so that one can make more of a profit, however, is not permitted.
Summary: If one cannot pray with a Minyan, one should pray at the time that the congregation usually prays. One is exempt from a Minyan in order to prevent a financial loss, or if one is not permitted by one’s employer to attend one.