Openvpn works fine after installation but then stops working after a while

[abbesh9]

This is very strange, I configure openvpn for my customer and it works fine for a few days and then it stops working. I can not ping the same devices I was able to ping just a few days ago. This has happened more than once and for the life of me I can't imagine what could be the cause. Has anybody seen something like this (the dynamic dns works fine and the IP address hasnt changed either).