About 10 years ago, I had a similar condition. I would experience a dizzy spell for 3 or so hours and/or block out momentarily. Sometimes I would just black out briefly with no warning. The worst part about it was that it took the doctors about 9 months to get me properly diagnosed. In the end, they determined I had something called neurocardio syncope, which is essentially a heart/brain communication issue. My brain would think I was losing blood pressure rapidly, and, thinking I had suffered a massive trauma of some sort, would tell my heart to nearly shut down, in order to conserve blood. Problem is, I was fine, and my heart slowing caused my blood pressure to drop, which prevented enough blood from getting to my brain, and I would black out.
The good news was that it was treatable with 18 months of heart medication, and I've only had a few dizzy spells since then.
Hopefully your doctor will be able to figure out what happened, and all will be well.