All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
If you had combined the two prayers without a valid excuse, then you are sinful because combining two prayers without a valid reason is one of the major sins.
Among the valid reasons that permit combining the prayers are travel, sickness, and the work whose doer is excused for abandoning the congregational prayer.
If you had combined those two prayers while you were a resident and not traveling, and you shortened them, in a way that you had prayed each prayer two Rak’ahs (units of prayer) only, then this prayer is invalid, because it is obligatory for whoever combines the prayers while he is a resident and is permitted to combine them that he must pray them in full, without shortening them.
Therefore, you must make up for those two prayers.
In addition to this, if you made a break between the two prayers when you combined them, then if you had combined them at the time of the first prayer, then there is a difference of opinion about the validity of the second prayer, and it is better in order to be on the safe side to repeat it. But if you had combined those two prayers at the time of the second prayer, then performing them without a break is not a condition in this case.
As regards combining the prayers for a sin, then this is forbidden. Going to the disco is not a reason for combining the prayers at all; rather, you must repent from these two grave sins: the sin of going to the disco and the sin of delaying the prayer until its prescribed time has elapsed.
Allah knows best.