You have many suggestions receiver. One is RESETTING the SIM900 modem, but no instruction for that. Guess I experienced the same problem as you firstly explained, namely that the AT command you put in the command window of the serial monitor did not respond with OK.
- Make sure setting the correct baud rate (ie. 9600 or 19200) according to your sketch.
- Rest by typing ATZ.
- Check for working by typing AT. The monitor should now answer with OK. Hereafter you can type any AT-commands.