Student or Learner
1) I was sitting alone and then all of a sudden i thought about my parent.
2) I was sitting alone and then suddenly i thought about my parent.
2) I was sitting alone and then my parent come to my mind.
A few changes
1 - Make sure "I" is a capital letter
2 - We don't usually refer to our parents in the singular. My mom, my dad, my parents.
3 - All of the sudden
4 - came to mind, not come to mind
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.