Yesterday, I had a fast but I forgot to read niyyah, After iftar I remembered that I did not rade niyyah, Will my fast is still vaild?
The Niyyah is a really essential for all good deeds and our Niyyah must always be good.
I didn't understand what you meant by reading Niyyah but if you had eaten Suhoor then you must have made an intention to fast.
But if you are sure that you didn't intend fast at all then your fast cannot be valid.
It was narrated by Ibn ‘Umar from Hafsah (may Allaah be pleased with her) that the Prophet (peace and blessings of Allaah be upon him) said: “Whoever does not have the intention of fasting before Fajr, there is no fast for him.” Narrated by Imam Ahmad, the authors of al-Sunan, Ibn Khuzaymah and Ibn Hibbaan
However, the Niyyah doesn't have to be verbal but it can be taken in the heart.
Prophet (peace and blessings of Allaah be upon him) said: “Actions are but by intentions, and everyone shall have but that which he intended…” (Narrated by al-Bukhaari, 1; Muslim, 1907)
So, you have to think whether you had taken the intend to fast or not and act accordingly.