First, pull over to a safe area of the shoulder. Once if it's safe to do so, get out of your vehicle and inspect the damage. Write down as many details as possible, including the time, date and exact location where the incident occurred. Next, inspect the windshield to determine if there is any glass missing from the windshield or if there are any pieces that may fall out as you continue to drive the car to your next destination. If the damage is not significant (and only if you can drive your vehicle safely), drive to your destination and start planning on your next steps.
Windshields are designed and manufactured for safety in case of accident. That is, they are designed to stay intact during collision as a result of their laminated safety auto glass construction (two pieces of auto glass with a layer of vinyl between). However, just like a bicycle helmet, your windshield will not offer optimal protection if it has been weakened due to damage. Thus, as soon as you notice damage to your windshield, it is in your best interest to have it repaired.
Generally speaking, windshields can often be repaired rather than replaced, though it does depend on the extent of the damage. If the windshield damage is less than 6 inches long or is in the shape of a star or circle, repairing is likely to be the choice you'll make. Even cracks can be repaired, though they should be clean and less than one month old to be considered good candidates for repairs.
In most cases, windshield repairs take approximately 30 minutes. Depending on where you live, the auto glass repair shop may even provide mobile service that can come to your location to perform the work. There are a lot of options, so continuing to drive around with a broken windshield shouldn't be one of them.