Is prayer valid after waking up and performing ablution if semen is found later without awareness of a wet dream or lust? And what should be done in this case?

You are not required to go to the bathroom after sleeping to check for the emission of semen. However, if you are certain you observe semen after returning from prayer, you must perform ghusl (definition) (ritual bath) even if you don't recall a wet dream. Your obligation now is to perform ghusl and re-pray all the prayers you performed before ghusl. One ghusl is sufficient for seeing semen twice. If you know the number of invalidated prayers, re-pray them. If you do not know the number, continue performing make-up prayers until you are reasonably certain you have absolved yourself of the obligation. The preponderant view, and that of the majority of scholars, is that re-praying is obligatory.