Some SLR cameras are prone to having the mirror slide off the front edge, especially if it has been stored in a hot place, like the inside of your car for any length of time. This is because there is no lip on the front of the the mirror frame to keep this from happening. Contax, and Yashica are two brands this is common on. The strange thing is, if you are using Yashica ML lenses on your Contax instead of the Carl Zeiss lenses (they share a common mount called C/Y), you may not know it is happening since the Yashica lenses have a different construction on the rear light baffle than the Zeiss. Which is kind of good if you're in a pinch, and your Zeiss lenses start interfering with the mirror, you can just switch to Yashica until you get a chance to fix the problem. If your Yashica lenses are hanging, then you're just out of luck.
It's fairly simple to fix. You need a locking cable release, a hair drier, a wooden dowel, and a running electric fan.
Step 1) Lock the mirror in the up position by applying the locking cable release to the shutter release socket. You will see how much the mirror protrudes from the end of the mirror support frame. It takes slightly more than 1mm of travel for it to interfere with the back of the Zeiss lenses, a little more with a Yashica. Leave the back closed, and lay the camera on its back in your work area. This is so you will not have to handle it much while the heating process is going on.
Step 2) Start warming up the base of the mirror with the hair drier. (Do not use one of the industrial heaters that you can use to heat shrink wrap tubing, as they are too hot.) You want to heat the mirror quickly, because the longer it takes, the more the heat will spread throughout the camera, and may cause lubrication migration, so use the High setting on the drier. Use the wooden dowel to push on the end of the mirror to see when it starts to loosen up, as you want to move it as soon as possible to keep the total heat build-up at a minimum. When it does start to move, work the front edge from one end to the other, ensuring it is straight when you are done.
Step 3) While the mirror is still locked up, immediately place the camera in front of the running fan to start the cooling process, opening the back of the camera to let more heat out. Leaving the mirror locked up will let the air flow through the mirror box, helping with the cooling, and holding the mirror in place until it sets (in case the glue wants to rebound). You should turn the camera around a few times while the cooling is in progress to reduce heat stress on the camera. I use a small Lazy Susan to facilitate in the process. Remove the locking cable from the shutter release after the camera has cooled a bit, so the prism/focus screen area will get some air. When the camera has completely cooled, check that the mirror now clears the back of your lenses.
Give it a day or so before you test the camera with film, just in case there is some rebound of the glue, checking thoroughly through many shutter actuation cycles that the mirror has not moved.
Waiting for the light