I've never heard of anything called "iso assistant". I can't even find anything called "iOS Assistant", a name which would make more sense.
With a jailbroken iPhone you're pretty much on your own - we can't help much in these forums, but try putting the iPhone into DFU mode and restoring it:
http://www.iclarified.com/entry/index.php?enid=1034
If that fails, then the jailbreak may have permanently disabled your iPhone. You'll have to look elsewhere to try and find any additional suggestions.
Good luck.