Q: Why am I encountering a Black Screen When Launching MWO?
It's likely that your video settings for MWO are set in such a way that the client isn't able to render correctly. Your video settings are stored in an attributes.xml file in your MWO Profile folder; removing it will force your client back to the default settings upon your next login, and you won't lose any important data on your account. However, it may force you to run through the Tutorial once more.
Head to your C:\Users\<your username>\Saved Games\MechWarrior Online\Profiles\<your pilot name> folder, and remove the attributes.xml file located there.
After you've done that launch MWO and log-in and see if this issue persists.
If the issue persists after running through these steps, please contact support at firstname.lastname@example.org