Progress to Ph.D. delayed due to COVID-19: How to convince advisor to let me finish degree without taking an extra year?
Very sorry to hear about your situation. I know of others, like yourself, who've had their research stalled as a result of the COVID-19 situation. I must say, I was taken aback shocked to hear about such inflexibility on part of your supervisor:

I asked about proposing a partially online experiment so I could gather data this summer, but she refuses to let me do any online data collection for my dissertation.

If you haven't yet already, then you should really have a heart to heart with your supervisor and explain to them how you're feeling. Try and see if it is, in fact, the case that they are fixed on this issue. Should collecting data online be a feasible means for you to collect data for dissertation and defend by the fall, it would be very unreasonable on your supervisor's part to not allow you to do this. So if your supervisor is firm on this position, I'd recommend consulting with your department's ombudsman or even head. It is definitely not fair that you should be held back if that is not a necessary outcome, and merely reflects your supervisor's research motivations.