I've solved this problem. This is the link:
How To Restore the Wubi Ubuntu Bootloader - How-To Geek
And one correction of the tutorial above. In this section:
It's not bcdedit /set {device_id} partition=X: , it's like this: bcdedit /set {device_id} device partition=X:
I don't have them in separate partitions. Since wubi doesn't work that way. Here is a quote from wikipedia.
I just want to make a some sort a path to Ubuntu installed.
I know that one possible way to get Ubuntu running under Wubi is to uninstall win8, and install win7. But I want to solve...