A restore should be done if powering your iPhone off and on doesn't resolve it followed by doing a reset, which is done by pressing and holding the Home button and the sleep/wake or on/off button simultaneously until you see the Apple logo and then release - ignoring the slide to turn off prompt when doing so.
Restoring your iPhone from your iPhone's backup should the next step if the first two troubleshooting steps do not resolve it, and if a restore from your iPhone's backup doesn't resolve it, restoring your iPhone as a new iPhone or not from your iPhone's backup should follow.
Has there been heavy cloud cover where you were driving over the last couple of days? If so, this may have an effect on GPS access.