i had the same Problem yesterday i fixed it that way:
put your /srv/var/ files where they belong (the files of your homepage).
And create a databse named ispconfig, in which you dump your backup of the old isp tables.
Then install ispconfig, and give him this database information. Isp regcognizes that there are webs customers etc and you can use thema as before
Plz keep your backups i can give you no garrantie
In my Case it worked perfect