If the backup is 3 months old then you are going to lose 3 months of data, a "backup" is basically a snapshot of the site at the time it was made, typically weekly backups are a norm because in the worst case you have lost 1 week worth of data. Daily are the best but can be costly in server space to keep enough of them to be "safe".
So YellowWebMonkey goes into using Akeeba kickstart, that will restore the site to that backup but you will lose everything else in the last 3 months, this is the only option to "restore" if that is the only backup you have.
If you have any sort of programming background or know someone who does you can "fix" the current site to get it working again, then speed up your backup rate to prevent a situation like this coming up again. This can be a lot of work but in some cases is the best option.
If you clarify on what is wrong with the current site, there is a possibility we can help with that.