It is not uncommon for Windows 8 to not boot after installing Ubuntu. To fix this you can use '
Boot-Repair' utility.
Now, this is a tricky part.
Did you erase/delete/format the Windows8 Partition? Its important you tell us how you installed Ubuntu. Did you use the option 'Replace Windows', or 'Alongside Windows' or 'Something Else'?
If you formatted/deleted windows partition then it may be possible to recover that partition but its improbable that you may recover all the files/data on it. Its worth a try considering the other option. You can have a look at
http://www.minitool-partitionrecovery.com/ (You will have to buy a licence to recover your partition).
The other option will be to re-install Windows 8 (as the recovery partition may not work). You can contact Asus and tell them that you accidentally deleted/formatted Windows8 partition and that you cannot recover it. They may offer you a better solution.
Once you get your Windows back, then you can re-install ubuntu carefully without affecting your Windows.
My two cents....
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