I think this is an easy fix you can make by removing the bottom plate and resetting the mirror latch arm.
However, if you do not feel confident removing the plate or working inside the camera, you should get it repaired by someone who has the experience.
When uncocked, the little black arm is at the bottom of its slot and the large silver lever is at rest as you see here:
When properly cocked, the black arm is at the top and is held by the silver lever as you see here:
In your case, most likely the camera was wound but the black lever slipped to the low position. To correct this, you must raise it and adjust the lever to hold it in place at the top. Most likely your lever is the early type which looks a bit different and is known for this problem.
Here is a video that shows the problem (but take the battery out first!):