After my check engine light came on i disconnected battery and after putting battery back in now the car will not start .what is it and what should i do?

Disconnect the battery (be sure to put a rag between the cable and any metal surfaces to prevent arcing). Depress the brake pedal after battery is disconnected, this will discharge any capacitors holding a charge in the electronics. Let sit 10 minutes to kill any memory data saved on computer in vehicle, then hook back up and start vehicle. This will clear any codes, or check engine lights to reset them. If there's an issue with the engine, the indicator will light back up and you will need to get the code checked. If the Check engine light comes back on , then you can take to local "Auto Zone or Advanced Auto Parts", not sure on Oriely's but they can plug in their code reader for free, to tell you what it is.

Test the alternator out put.. First check battery for voltage.. should be 12v or slightly better.. Start engine and check the battery again.. and it now should be 14= VOLTS.. IIF ITS NOT 14+THE ALTERNATOR IS POSSIBLY BAD.

Will it jump start? What shape is the battery in?

Much of the time this is a faulty alternator. It may be charging fine, but when the engine is off may still have a draw on the battery. To start with, you need a test light. Disconnect the negative battery cable. Connect the test light lead to the negative side of the battery and touch the test light probe to the negative cable. If you get a light you have a draw on the battery. that means the battery will be drained in a short time. Disconnect the alternator. If the light goes out it's a faulty alternator. If the light stays on, start pulling fuses one at a time until the light goes off. Do not pull all of the fuses at once! Pull one, if the light doesn't go off, put it back. When the light does go off, whichever fuse it controls is the problem. It's really not that difficult. Good luck.

no in most cases on them its the oxigen sencore

Check Engine Light can be reset by disconnecting the battery terminal and and putting it back again. Make sure you have the code for the radio before you disconnect the battery terminal as the radio can get locked. You can get the radio code from any dealer.
Another way to rest the check engine light is by using an error code checking meter. You will have to purchase one.
It is always better to first get it checked from a mechanic for the reason of the light to come on before you reset the light as it might be an indication of a serious engine issue.

As I know disconnecting battery cable cannot reset engine light.
Engine light on caused by a fault code remained in the OBD system. You have to clear the fault code to reset engine light.
But before that, I suppose to have your car checked first to make sure there's nothing wrong.

Good luck.

Sounds like battery or battery cable problems to me.
If the battery is bad, you cannot trust ANYTHING the computer is telling you. They tend to set all kinds of codes when the battery has a shorted cell. (including fuel pump circuit and transmission system codes) They get "delirious" from malnutrition. You need to have your charging/starting system thoroughly tested.

Please read these articles:
My Car Won't Turn Over

What Else Could Be Wrong?

The Check Engine Light needs to be reset unpluging the Battery won't clear it go back to Auto Zone see if they will reset it for you.Good Luck.

p1684 means the battery was disconnected within the last 50 starts!!
this means that the light should go off, if it continues to set that same code i believe it's because the battery voltage is too low!!
so buy a new battery and that should fix your problem!!:-)