The types of evidence important for your case often depend upon the type of accident you experienced.
The following generally help in brain injury claims:
Police reports
Store incident reports (for slip and fall accidents)
Accident scene photographs, videos, and possible media reports
Witness statements and the contact information for eye-witnesses
The medical costs and reports related to your injuries
Your lawyer can inform you regarding what types of evidence they may need to prove negligence. If you do not possess any evidence, they can investigate to find what they need for building your case.
Do not let a lack of evidence stop you from contacting a brain injury lawyer as soon as possible. The details of your accident play an important role in what your lawyer may or may not need to pursue compensation on your behalf.