To miss a Fardh Salah by not performing it in its prescribed time is a serious offense. If a Salah is missed, abundant Taubah and Istighfar must be made, followed by a firm resolve not to miss a Salah again. By repenting sincerely, one shall be forgiven for the sin of delaying the Salah from its fixed time.
However, in the unfortunate situation that Salah was not performed in the prescribed time, it is compulsory to make up and perform the missed Salah. This is called Qadha.
Taubah does not exempt one from performing the Qadha. If one does not make Qadha of the missed Salah, the sin will remain.
Rasulullah (Sallallahu Alaihi Wasallam) was once asked about a person who misses Salah due to sleep or ignorance. Rasulullah (Sallallahu Alaihi Wasallam) replied that the Kaffarah (way out and expiating of the sin) is to perform the Salah as soon as he remembers. (Sunan Nasai; #613)