It may be a calcium deficiency, the demand from calving and producing all the milk for the calf can kill them.
I had one give birth on Nov 4th and on the 5th she was laying flat on the ground and could barely open her eyes. I had the vet out and he gave her an IV of calcium and she was ok within 30 minutes. I held her head up with my knee as he gave her the IV and as she got stonger he said smack her and make her get up. I slapped her a little to soft and he said hit her a good one and make sure I have an escape plan before she gets up (Brangus Cow)
she popped up and I took her baby back to her and all was fine then.
I was told to make sure my free choice minerals had calcium supplement in it from now on. I use chicken litter for my pastures and that is one negative in them getting enough calcium.
learn something new all the time.