You might need to supply some more information, in order for someone to help you...
For example, what's the client? (Windows PC, Android phone etc).
What software are you using on that client? (built-in, Draytek SmartVPN etc.)
What type of VPN is it? (L2TP/IPsec, SSL etc)
Can you connect at all? ... do you get an IP address? ... can you ping the router?
What do mean by "I cannot access any of the PCs on my LAN when on the VPN"? ... what services are you trying to access on the PCs? (File sharing, Web Server, UPNP Media etc etc).
Can you ping them? (by IP address and/or name).
