Why did my bill go up 75%?

Were you in some kind of promotion that may have expired.  Compare your bills line by line and you'll find what changed.  

Compare bill to the prior one. If that doesn't answer it, callback and speak to a different agent.

This is a public customer to customer forum. If you were to share the line items of the bill, perhaps we could help clear this up. But for account access you would need to speak to customer support.

75%?

Were you an ACCESS account that either changed speed tier losing the ACCESS pricing….

Or do you a TV product on the account? My understanding is the new company that purchased the tv entertainment business from ATT in July may no longer being offered discounts as the tv service is now separate from the internet / home phone. If this is the case, the bill would increase but 75% would be difficult….

care to share some numbers?

I agree that a "75% increase" doesn't sound right. Either that is way off or there is more info needed.