Hi there. This happened with my father's 7D when it hadn't been used for a few months. I love this camera and found it very frustrating. My solution, whether it's temporary or not (I suspect it is, as this is an ongoing issue with the 7D) is to take the lens off, make sure your battery is charged and set the camera to sensor clean mode. You will see the mirror flip up. When this happens, there is still a curtain over the sensor. Leave the sensor clean mode on for a minute or so, then switch off. Repeat this until you see the curtain move and the sensor is visible.
I did it a few times and eventually the curtain snapped back, the sensor became visible and pictures are now able to be taken without issue. I've not had a problem since. This may just let you use it as long as it's not for something utterly crucial where reliability is paramount.
A true photograph need not be explained, nor can it be contained in words.