At the beginning of the year I sarted seeing a girl. But she ended it befire it got seriouse.
Recently I started thinking about her again (I don't no why), then a work colleague asked me about her this week, and I'm thinking about her more, now I really don't no what to do.

What suggestion do you have?

At the beggining of the year i sarted seeing a girl. but she ended it befire it got seriouse.

"on_myt_own" there really is no question,here,about what you should do!

This is found,in your question when you say,that,the girl ended it before it got
serious.

DO NOT in any way try to get in contact with this girl. After all,she's made her wishes
Quite clear (by ending it) NO CONTACT

Keep living your life, and stay out of hers.

I know you might like her but you'll forget in time. If you did pursue her- you'd just get hurt because she doesn't want what you want.