What error messages are you receiving when you try to ping? Finding out the error, and more importantly what it means will be the first step in locating the problem.
You indicate that the network process is running, but you need to restart it which corrects the problem. This suggests that part of the networking system has terminated, probably with an error code. Look in your log files, such as syslog, for such an error or termination code. You may also want to look at the output of dmesg for an error associated with your network hardware.