How handy are you? If the lever has fallen down it is more than likely one of the springs inside the indicator mechanism has broken/fallen off. Mine would not stay in the "up" position and it required a bit of work with a dremel.
Disconnect your battery because there is a lot of amps running to the steering column. Pull the steering wheel off. It's really easy to make a steering wheel puller out of flat bar and 2 6mm high tensile bolts. Don't try remove it any other way cos you will wreck too much and break your hands and temper in the process. You will have to do this to get to the turn signal anyway. Two screws hold the plastic housing and the turn signal stuff to the column. At this point you will either replace the broken part or choose to repair it.
There are 4 (?) screws that hold the halves together. Once the halves have been VERY carefully opened so you can see where the springs and bits go you can make your decision depending on what is broken/loose. If you can pull it apart without bits flying around, take pics so you can put it all together again.
With a bit of luck it is just one of the 2 springs that has fallen off. I'm working from memory here and repaired my light switch at the same time but I think I remember most of it. Good luck
..............Sorry I have a habit of relying before I read properly. Do you have the original wheel on your car? If you do, you are going to have to pull it off to investigate. There is a collar with a pin that fits into your steering. It is designed to restore the indicators to the off position when the wheel is turned to the straight position. If the pin is broken, or the collar is not turning it may be the cause of your problem. If that seems ok, then you may have to open the indicator mechanism to find the problem.