MY MEMORY CARDS NO LONGER WORK IN MY SONY DSLR A100 CAMERA?

Well, the good news is that there's probably no fault with the camera itself. It could be something as simple as the CF card being pushed in the wrong way. A look through the user manual will likely help you get to the bottom of your issue:
Sony Alpha DSLR A100 manual

the problem is with the CF card nit the camera
get a new card and your good to go

The format command is in the playback 1 menu.

Press the MENU button. Use cursor-left/right to select Playback (right-pointing triangle) and press the button at the center of the cursor pad. Use cursor-left/right to select "1" then use cursor-up/down to select "Format".

If you need a manual you may download a copy from here.

It's not the memory card, it's the sensor. Contact Sony.

not all memory cards fit sony alpha perhaps you have bought wrong cd.try using your sd slot

Is the new card "cheap"? If so, it may be too slow to keep up with your camera.

Its a minor problem. You can check form website the address of nearest service station.

card slot pin is stuck or broken

Click Sony Support and download required instruction manual of Sony DSLR-A100

Hi! I just had the same problem with my Sony A100, when I tried two different CF cards. I was getting the same error message as reported above: "Unable to use card" then "Format?" then that format fails. Thinking the camera is broken, I checked online camera repair sites and also called Sony directly. Sony's flat rate, and that of PrecisionCamera is $228 to fix CF card failure. Then my daughter said, "the CF card in my room works fine in the camera" - and lo and behold - it does. The CF card that works is brand PQI and is marked Hi-Speed 60 2Gb. The cards that do NOT work are marked SanDisk Ultra II 1.0Gb and SanDisk 128 Mb. So my advice is, before you give up on the camera as broken, try a number of different CF cards first.

The second cause of CF failure is if someone has pushed a CF card in the wrong way. That can bend one or more of the tiny metal contact pins in the CF socket in the camera. Look carefully to see that all the pins are straight up. [I don't know how one can straighten these pins, or if the whole socket would need to be replaced...but at least you can determine the cause of the CF failure.]

Best regards - Michael