180 days from the date of the injury/damage, served on the appropriate official in writing containing the specific information required by the statute. If you are suing Montgomery County, you must personally serve (or mail by certified mail, return receipt requested) the County Executive. The notice must state the time, place and cause of the injury in sufficient detail to apprise the County of enough information to understand and investigate the claim. You should use a lawyer so as not to prejudice your claim.
You should get a lawyer right away. $50,000 in damages is worth pursuing the right way. As pointed out, there are special rules for claims against governmental entities and lawyers are experienced in meeting those requirements. Good luck.
Generally in Maryland, when suing a county, you must notify the county no more than 180 days following the event. If the suit is against the state, the state must be notified within one year. However, the rules with respect to contract disputes and tort cases may differ, and it is important to make sure that you have a firm grasp on which type of case you are pursuing. From the sounds of your question, it appears as though you are looking at a tort action, but, again, it pays to make sure.
Also, be sure to name the correct defendant if you are filing suit. Generally, the correct defendant is the actual tort-feasor, NOT the county. Finally, be sure that, if you do file a Complaint, you affirmatively allege that you properly notified the county. A recent case has held that this is essential.
Causes of action against a county can be quite tricky. It probably makes sense to at least consult with an attorney to be sure you are on the right track.