What should one do if one wakes up when the time for Asr prayer begins, having slept through Dhuhr prayer? And should I combine the two prayers upon waking, or what?

Reviving the nights of Ramadan and striving in worship hold great merit, especially in the last ten nights. However, the most beloved act by which a servant draws near to his Lord is the performance of obligatory duties at their prescribed times, as stated in the Hadith Qudsi: "And My servant does not draw near to Me with anything more beloved to Me than what I have enjoined upon him."

Therefore, one must be diligent in performing the obligatory prayers at their prescribed times. If you miss a prayer due to sleep, you must pray it as soon as you wake up, at any time, for the Prophet, peace and blessings be upon him, said: "Whoever forgets a prayer or sleeps through it, its expiation is to pray it when he remembers it." So, if you wake up at the time of 'Asr, you must pray Dhuhr first, and then 'Asr immediately.