I sang a part of a song the day before yesterday,it didn't go that well as my singing other songs did so I thought that song didn't suit my voice. I sang that same part of that song today and it was amazing,what caused this and what do I do to maintain?

I don't know without hearing you, Clint! Our voices are different from day to day due to usage, hydration, stress, sleep, allergies, and more. I encourage you to warm up as part of your practice to help build consistency in your voice, and over time, you'll get to know your voice better.