What version of iMovie are you using and what operating system? I am assuming iMovie 10 but don't know what version.
Possibly a corrupt preference is causing the share options not to appear. Try opening iMovie whild holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if you can share out. (Note, that sharing directly from iMovie to You Tube often is problematical, due to Google not yet upgrading its online uploader to keep pace with Apple's recent upgrades. More reliable to share to your desktop and then drag the movie into You Tube's upload window.)
If deleting preferences doesn't help, try doing an Edit/Select All, Edit/Copy, of you media in the project and then Edit/Paste it into the timeline of a newly created project. Then try to share out. This is to rule out that the issue is confined only to one project.
Another diagnostic tool is to open iMovie in a new library and see if you can share your project from there. To do that, hold down the Option key while opening iMovie. In the box that appears you will have the option to create a new libray in which iMovie will open. Then copy your project into the new library and see if you can share out.
Sometimes booting up in Safe Mode can clear out conflicts that cause issues. Refer to the Finder help menu for directions on how to boot up in Safe Mode. After booting up, wait until everything loads and then shut down and reboot in normal mode. See if now you can share out.
All else failing, try redownloading iMovie. Drag your iMovie app from the Applications folder into the trash, but don't empty the trash. Then sign in to the app store and open your account by clicking on your name at the bottom of the app store sidebar. When you account opens you will see iMovie listed among your purchases. Download it from there. If unsuccessful, drag your old iMovie from the trash and replace it in the Applications folder.
-- Rich
