My parents live in Dubai and had a VPN for VoIPing, but recently the VPN stopped working. I'm terrible at networking and don't really know how all of this works, but is it possible for ISPs to block particular VPNs ?
yes, at both the network level (by blocking IP ranges belonging to VPN services) and protocol level (ie. known handshake sequences for VPN software). but both can be easily bypassed with a few google searches.