"I had come to talk to you but you were not there."
Translation:J'étais venue pour vous parler mais vous n'étiez pas là.
Most verbs use "avoir" but some use "être" when forming compound verbs. Reflexive verbs and the following intransitive verbs use "être": aller, arriver, descendre, devenir, entrer, monter, mourir, naître, partir, passer, rentrer, rester, retourner, revenir, sortir, tomber and venir. But when some of these (descendre, entrer, monter, passer, rentrer,retourner and sortir) are used transitively (with a direct object) they are conjugated with "avoir".
I'm still having difficulty wrapping my head around when to use "etre" or "avoir" in these sentences. I found this great resource. Hopefully it will help others who may be having difficulty with when to use which verb. http://www.french-linguistics.co.uk/grammar/avoir_or_etre.shtml
The verb "venir" is one of 14 verbs that require "être" as the auxiliary verb (not "avoir"). http://french.about.com/od/grammar/a/etreverbs.htm In addition, all pronominal (reflexive) verbs use être.
The subject is "vous", so the verb's conjugation should be "étiez" (not "étais" : je or tu).
You can use "y" as a pronoun replacing a place already mentioned:
- j'étais venue chez vous, mais vous n'y étiez pas.
In your sentence, no place has been identified, so "y" should not be used.