If your accident might have been even partially caused by a government entity or employee -- the city, county, state, or federal government, or any public agency or division (a city bus or a school district, for example) -- you must file a formal claim within a short time after your accident. This period of time usually ranges between 30 days and one year, depending on your state. If you fail to file a claim within the time limit, or fail to include required information in your claim, you may forever lose your right to collect compensation.
To find the time limit for your state, call your city or county attorney's office and ask. Although they may be the ones defending against your claim if you file it, they are under a legal obligation to give you correct filing information.
You can also find a complete list of time limits, plus instructions on how to file a government claim, in How to Win Your Personal Injury Claim, by Joseph Matthews (Nolo).