Bake does not work but broiler does.

OK, it is likley that the oven ignitor isn't working and needs to be replaced. To verify if this is the problem, when you turn on the oven to BAKE and set the temp at say, 350. Watch through the window, (or you can have the door open). Look at the bottom, back, center and see if you see a bright orange glow appearing under the bottom panel.

If you don't, that is why the oven won't lit the gas. There is an ignitor for both the broiler burner at the top and for the bake burner in the oven compartment.

The reason that the temp display reverts to 100 degrees is because that is the lowest default temp setting for the display and until the oven temp actually starts to increase, it won't change. So the temp display is working correctly for the BAKE function.

Here's a line drawing showing the part you may need. Ignitor Part # WB13T10045. The good news is that your oven uses the same ignitor for both the broiler and bake burner tubes, so you could swap them if you wish to confirm that is the problem before you go and order/buy a new one. These aren't cheap at $91 bucks, but you can shop around the web for a better price if you determine you need one.

Sometimes the ignitor burns out half way through but because it still lits/glows, people think it's OK, but it won't get hot enough to signal the gas valve to open and allow gas to ignite and get the oven cooking.
http://www.searspartsdirect.com/partsdirect/getSubComp.pd?modelNumber=JGS905SEK1SS&productCategoryId=0124250&brandId=3272&modelName=GAS-RANGE&diagramPageId=00001&componentDescription=GAS%20&%20BURNER%20PARTS&documentId=G6090295&backToLink=Return%20to%20Sub%20Components%20list

Hope this helps you out and remember to disconnect the oven from the electric power before performing any repair work. Best regards!