Cannot connect to VPN?

I can't seem to connect to the VPN servers (L2TP or PPTP) through my mac since recently. I've tried different networks(Home Wi-Fi/Mobile Broadband Dongle) nothing seems to work, the strangest thing is that even when I share my internet connection to my iPhone, the iPhone cannot connect to the VPN service as well. So I'm assuming it has something to do with my mac's side. Any idea on how to reset my network settings or to fix it?