If after following these steps you still have problems, please try the following:
1. Restart Anki
Please close Anki, then start it again.
If you are unable to close Anki because of an error message, you can either terminate Anki using your task manager, or restart your computer. Anki saves periodically, so you should not lose more than a few minutes of work in most circumstances.
If the problem does not occur again, you can skip the following steps.
2. Check add-ons
Please close Anki, then start it again while holding down the shift key, which will disable any add-ons you have installed.
If the problem goes away when add-ons are disabled, you can narrow down the culprit by disabling one or more add-ons, restarting Anki, and seeing if the problem goes away. If you have lots of them, you can select the top half of your add-ons in the Add-ons screen and disable them. If after restarting Anki, the problem is still there, try the other half instead. Once you’ve found the half that is causing the problem, divide that half into half again, and repeat. You should be able to narrow it down in around ~6 steps that way.
Alternatively, if you don’t need all the add-ons you have installed, you could start by disabling all the ones you don’t need, and seeing if that helps. Repeat the process trying different add-ons, until you discover which one is causing the problem.
If the issue turns out to be an add-on, please report the issue to the add-on author.
3. Check your Anki version
These days, Anki shows its version in the error message. If you're using an old Anki, you can find the version you're using in the Help>About or Anki>About menu.
If the version you're using is not the latest 2.1.x version published on https://apps.ankiweb.net, please close Anki, install the latest version, and then start Anki again to see if the problem has gone away.
If you're using Linux, please make sure you can reproduce the error using the packaged version on the Anki website, as distros often distribute broken versions.
4. Check your database
After restarting Anki, please try the Tools>Check Database menu item to make sure your collection doesn't have any problems.
5. Restart your computer
Sometimes restarting your computer may help.
6. Change the Video Driver
On Windows and Linux, crashes and display issues can be caused by the video driver. Changing to a different video driver in the preferences screen or via the gldriver file may help. On Windows, make sure you try all three options.
7. If the problem remains
If you're confirmed you are using the latest Anki version, and are still receiving errors when starting Anki while holding down the shift key, please report the problem, including the next error you receive in your post.