You have several options some of them require more technical knowledge than others. If your computer is not booting, and you don't have the repair disks your options are limited. For the non-technical, take it into a repair shop or the Geek Squad and they should be able to fix it for you. Be aware that they don't always preserve your data and could charge you a chunk of change.
For the more technical user, I would first back up all your data, you can create a Linux Bootable CD or a flash drive and access all your files from there and copy them to a DVD or another flash drive, network location, the cloud, etc.
Although I don't condone illegal activity if you own a legitimate copy of Windows Vista (and lost the disk) you could download an ISO from just about any torrent site.
In a quick search I also found the link below which advertises free Vista recovery disks.