Every cycling accident case doesn't end up having eyewitnesses but that doesn't mean that you can't prove your case. The testimony of you, the cyclist becomes very critical at this point. And, it's very important that you know what happened and can explain in great detail what happened.
Many times beyond what you may say and what you know and observed is going to be considered circumstantial evidence. This would be where was the damage on your bike, were there marks on the road, was there a debris field where things might have been knocked off your bike and what were your injuries. All of this needs to align to what you are saying happened. All of these things can be used to prove your case even though you don't have eyewitnesses.
Though it's important to have eyewitnesses, you can still prove an accident case without them.