Before powering on, Remove the SD card if there is one inserted.
Power up the phone and see if the Android OS boots.
If you have installed a custom rom to the phone then while it is installing and booting you must remove your SD card also.
If it fails to boot then you will have to resort to a factory reset of the device.
If have not backed up your contacts, and programs to a SD card or Sim. then they will be lost when a factory restore is initiated on the Samsung device.
Hold the power button in and the volume up key on the device at the same time for about 10 seconds and you should be presented with a list of options to select.
From clearing the cache of the phone, to a factory restore.