Is fasting valid without praying?
Both praying and fasting in Ramadan are pillars of Islam. Both are therefore also mandatory to perform. But what about when you fast but do not pray?
Praying the five daily prayers is the second pillar of Islam. Leaving this is a great crime in Islam. It is according to the correct opinion an act of disbelief to leave prayer when one is consciously abondoning it.
Thus, Muslims should perform their five daily prayers without being negligent. When a person abandons the prayers deliberately without having a valid reason, then all good deeds will become worthless.
So the fasting is not valid if you do not pray, both in Ramadan and for voluntary fasts.
Start praying again in Ramadan
Allah does not accept the prayers of a person who abandoned prayer (whilst knowing it is disbelief to do that) and did not repent from those abandoned prayers.
If the Muslim was unaware of the ruling of leaving prayer, then he is excused due to ignorance. He should not leave the prayer after coming to know about the ruling.