You are obviously using old versions of Joomla & VirtueMart. Old versions are liable to be hacked if regular attention in not paid to security, but your problem may just be that the host has updated the server software (eg PHP version) so that the old software no longer works properly. It is likely that the time has arrived when it is necessary to update the site to current versions, most recent stable versions to use are Joomla 2.5.18 and VirtueMart 2.0.26d. Migration to current versions is quite a big undertaking, but after that, future upgrades should be easier.