I couldn't fix it on my phone using ES File Explorer. After I found the apk, I tried reinstalling and it would start, but I got a message that said "Application Not Installed" or something like that.
I kept searching forums and found this:
There no youtube app on my nexus s 4g and it say it downloaded? - Google Mobile Help I tried it and it worked.
This is what is says:
"If you're stuck with a Youtube app that will not open/launch and is not present in the app drawer out of the box you can try the following:
- Go into Manage Apps --> All
- Google Partner Setup
- Clear Data
- Reboot
Upon reboot you should now see the Youtube app present. You should now be able to update in the market as well (the button is inactive prior)."