Transmission is slipping out of gear

I was at a stop and went to turn onto a road (a little quickly cause there was traffic coming) and the car slowed and stopped going forward like it was in neutral. The gas gauge dropped immediately. I pulled off to a side street and turned it off. When I turned it back on it was fine. I checked the fluid and it is okay. Just fix the drive shaft, check engine light (x3), fuel pump and brakes on the car three months ago.