Without further information, I can only speculate.
In order to fight spam sent by infected computers some ISPs have chosen to block port 25.
This can simply be done by a packet filter dropping all packets with the destination port 25, effectively blocking access to all other mail servers out there which are listening on this port by default.
It is possible that ISP let packets through to his own mail server, only. This way he controls what mail is going out. Your ISP might have been blacklisted as a spammer network and has taken this step to get delisted again.
It is a drastic measure, as it not only blocks access for spam, but also legitimate access to other mail service provider.